In a quiet fund-raising effort, Ning has raised $15 million more, a round that is valuing the social networking start-up at an eye-popping $750 million valuation.

In its last fundraising $60 million round a little more than a year ago, Ning was valued at $500 million.

The money for this fifth Series E round comes from a Silicon Valley venture firm, Lightspeed Venture Partners.

The Palo Alto, Calif.-based Ning, founded by well-known entrepreneur and Ning Chairman Marc Andreessen and CEO Gina Bianchini, confirmed the funding when contacted by BoomTown. It was not actively searching for funding.

Other investors in Ning include LinkedIn Chairman Reid Hoffman, Legg Mason, and Allen & Co.

The additional funds raised bring the total garnered by Ning to $119 million.

Ning is a platform aimed at offering customizable tools that let users create their social networks about their interests, such as for fans of the movie “Twilight.”

Ning puts online ads on the sites, using Google (GOOG), and is also working on its own advertising platform. It also offers an array of other services and is planning more soon, such as a virtual-gift offering.

Founded in early 2007, it currently has 29.3 million registered users, who are using 1.3 million social networks, and it is adding one million registered users every 15 days, said the company.

But not all its social networks are active, and Ning’s monthly unique visitors are lower, according to various surveys, at about six million in the U.S.

Ning is one of Andreessen’s angel investments, although he recently raised $300 million for a new venture fund he is running with longtime investing partner Ben Horowitz.

In an interview, Bianchini said the goal was to become an even bigger platform for building social networks and the money would be used for possible acquisitions and other strategic options, attracting more talent and also to offer its social networks more tools to be discovered.

“It’s clear to me when you look the market…there needs to be a place for people to express their interests and passions,” said Bianchini, who noted Facebook and Ning do not necessarily overlap. “We want to be the social network for interests and passions online.”

Lightspeed’s Ravi Mhatre, who led the investment for the venture firm, said it was due to a lot of reasons, although he noted that the effort was not a traditional fund-raising effort, but more of an interest by Ning in adding a top VC to its investor pool and by Lightspeed in Ning’s hyper-growth.

“The growth at Ning has been massive in the last year and, combined with the quality of the team and seeing that kind of momentum, it worked out well for us both,” said Mhatre.

BoomTown’s most favorite part of the Yahoo takeover circus?

The dueling letters, of course! How the lovely practice of missives has fallen out of favor, as soulless emails have grown in use.

Well, not in the land of hostile takeovers!

So here’s our decoding of billionaire investor Carl Icahn’s thankfully brief letter to Yahoo’s Chairman Roy Bostock, informing Yahoo (YHOO) he begins bombing in five minutes.

Icahn wrote:

Carl C. Icahn
ICAHN CAPITAL LP
767 Fifth Avenue, 47th Floor
New York, NY 10153

Dear Mr. Bostock:

Translation: Who are you? Whatever. I regret to inform you, but I eat wimpy Chairman of the Board types like you for breakfast.

Icahn wrote: It is clear to me that the board of directors of Yahoo has acted irrationally and lost the faith of shareholders and Microsoft. It is quite obvious that Microsoft’s bid of $33 per share is a superior alternative to Yahoo’s prospects on a standalone basis. I am perplexed by the board’s actions. It is irresponsible to hide behind management’s more-than-overly optimistic financial forecasts. It is unconscionable that you have not allowed your shareholders to choose to accept an offer that represented a 72% premium over Yahoo’s closing price of $19.18 on the day before the initial Microsoft offer. I and many of your shareholders strongly believe that a combination between Yahoo and Microsoft would form a dynamic company and more importantly would be a force strong enough to compete with Google on the Internet.

Translation: Here I am stating the glaringly obvious. But don’t you like my use of self-righteous and indignant words like “unconscionable”?

Nonetheless, I must ask: What are you smoking over there on the Left Coast?

When someone dangles more than $40 billion to anyone on Wall Street, we’d throw our mother under the wheels of the bus if we needed to to get it. Frankly, we would do it for $12.43.

In any case, your break with reality is my golden opportunity.

Icahn wrote: During the past week, a number of shareholders have asked me to lead a proxy fight to attempt to remove the current board and to establish a new board which would attempt to negotiate a successful merger with Microsoft, something that in my opinion the current board has completely botched. I believe that a combination between Microsoft and Yahoo is by far the most sensible path for both companies. I have therefore taken the following actions: (1) during the last 10 days, I have purchased approximately 59 million shares and share-equivalents of Yahoo; (2) I have formed a 10-person slate which will stand for election against the current board; and (3) I have sought antitrust clearance from the Federal Trade Commission to acquire up to approximately $2.5 billion worth of Yahoo stock. The biographies of the members of our slate are attached to this letter. A more formal notification is being delivered today to Yahoo under separate cover.

Translation: Gordon Crawford of Capital Research has me on speed dial, for your information. Also, you don’t have Legg Mason’s (LM) Bill Miller to kick around anymore!

Thus, like the good-only-for-pate goose you are, I am going to force-feed the Microsoft (MSFT) merger down Yahoo’s gullet on my terms and remove you from any and all decision-making.

I have bought a ton of your shares, I am going to buy more, I am checking with the Feds.

Check, check, double-check!

And, as an added SuperPoke, my largely unimpressive dissident board slate includes Mark Cuban, whose Broadcast.com Yahoo overpaid for in the last Web 1.0 bubble, giving him the ability to be the loud-mouthed, but highly entertaining owner of sports teams and a dancing fool, I might add, to always remind you that nice guys do finish last.

Nice guy? That would be you. Finish last? You again!

Icahn wrote: While it is my understanding that you do not intend to enter into any transaction that would impede a Microsoft-Yahoo merger, I am concerned that in several recent press releases you stated that you intend to pursue certain “strategic alternatives.” I therefore hope and trust that if there is any question that these “strategic alternatives” might in any way impede a future Microsoft merger you will at the very least allow shareholders to opine on them before embarking on such a transaction.

Translation: Make a move to kiss up to Google (GOOG) any more and I promise I will slap you back to last Sunday, instead of just having a hissy fit like Microsoft CEO Steve Ballmer.

As to AOL (TWX), I have three words, if you try to merge with that dog: An Overwhelming Lawsuit.

Icahn wrote: I sincerely hope you heed the wishes of your shareholders and move expeditiously to negotiate a merger with Microsoft, thereby making a proxy fight unnecessary.

Translation: Wait, I have three more words: Make my day.

Icahn wrote:

Sincerely yours,
CARL C. ICAHN

Translation: First, I am adult who uses all caps when writing my name, as opposed to Yang, who signs letters like a toddler, like this: jerry.

Also, I am not sincere in any way whatsoever and never have been in my entire life.

Except that one heart-breaking time when I was but a wee billionaire activist investor: Rosebud!

And here we thought Yahoo was the champ at dithering.

Not so, it seems, as Microsoft CEO Steve Ballmer mulls over the many options, each of which the software giant’s execs have cavalierly raised and dismissed publicly in the last weeks.

We know it is a big deal, but Ballmer’s ruminating is starting to make Hamlet seem decisive.

To be fair, none of the choices are winners–at this point, it’s more like a digital version of Scylla and Charybdis.

For those who have not cracked Homer’s “Odyssey” since high school, it’s one of the more vexing challenges that Odysseus faced on his long trip home.

The basics: A narrow strait. On one side a sailor-eating monster called Scylla and on the other a sailor-sucking whirlpool. Thus, a need to maneuver away from the death-to-all whirlpool means you have to sacrifice a sailor or two to Scylla.

In other words, you have to lose something no matter what.

And that is exactly Ballmer’s dilemma in regards to Yahoo.

If he pays more, it adds billions of dollars to the cost and kind of makes him seem silly for saying he would lower the price.

It also begs the question of why Ballmer did not do this sooner to get this deal done more cleanly. And now its $32 to $33 rise, which would have been welcome even a week ago, seems too cheap, with Yahoo shareholders now wanting over $35.

If Ballmer offers less, goodbye big shareholders like Legg Mason, who will likely become recalcitrant to hand over the company after a financial hair cut.

If he walks away, Ballmer risks losing momentum in his quest to win in the online advertising game versus archrival Google (GOOG).

In addition, if Yahoo stock drops enough, other players will enter the picture. Also, the Google threat to be Yahoo’s ad partner–which is in the works, whether illegal or not–has become inevitable and has been ironically finally consummated because of Microsoft’s unsolicited bid.

And, finally, if he stays in it without resolution for much longer, Ballmer risks declining morale of his own employees. Make no mistake–especially Microsoft’s important software engineers–increasing numbers of the company’s employees do not support this deal now.

So to avoid the dreaded whirlpool and geek-eating monster, Ballmer seems to have one option now–pay up if he really wants the company and thinks online advertising is a mission-critical aim of Microsoft.

And, if not, walk away and keep on walking.

BoomTown feels like a digital postal carrier today, delivering a small message each for Microsoft (MSFT) CEO Steve Ballmer and Yahoo (YHOO) CEO Jerry Yang from some of your bigger shareholders–some of whom own you both, in fact, and with whom we like to check in with from time to time to gauge their mood:

Steve: Greetings! Well, not greetings, exactly, since many of us are still really annoyed by the mean letter you sent to Yahoo two weekends ago.

Actually, we would not have minded a mean letter directed solely at Yang and the board.

In fact, if you had just focused on Yahoo’s lack of cooperation and your frustration in wanting to start negotiating and reiterated how valuable Yahoo was to you, that would have given us cover to phone up Yahoo and complain about inaction.

Instead, you threatened a price drop, which is like delivering a Kung Fu Panda blow right to the collective windpipe of big shareholders and makes it impossible for us to do anything but complain about Microsoft.

Which is precisely what Legg Mason’s Bill Miller did like clockwork, of course.

“The problem is Microsoft blundered with the letter this weekend,” Miller said flatly in an interview in The Wall Street Journal. “Telling the shareholders you’re going to take something away from them is not a way to get their support.”

So, now to assuage us, you probably have to raise the price. A poll of those BoomTown talked to said $2 more would do it and $3 would be a clincher.

Our advice: Lob a call into Miller and also Capital Research & Management and all the other big owners of Yahoo shares and do a little sweet-talking.

Dear Jerry:

Salutations! Well, not salutations, exactly, since many of us are perplexed at what exactly is the plan.

OK, we like all the activity of late, as it is keeping the pressure on Microsoft. But we are deeply dubious of the efficacy of all the various plans.

While we would grudgingly accept a union with AOL, with an investment from Time Warner (TWX) and even a stock buyback, we are nervous that it could be a disaster.

Most of all, there is the question of leadership and who would run this obviously hard-to-manage organization. We are not so sure that anyone in either team is up for it, and we just cannot imagine making that call to Steve Case and Bob Pittman to reassemble the old band.

And, while we love the idea of Google adding $1 billion in cash flow to the bottom line via an outsourcing deal to take over search-ad monetization, it’s a risky move fraught with regulatory questions, potential legal quagmire and increased aggression from Microsoft.

In fact, given how clear the Microsoft option is, especially if the price goes up or it switches to an all-cash deal, we still maintain that the most likely outcome is that we will support a richer Microsoft bid, since it presents us with the most clarity.

Or, as Fergie sings in her delicious “Big Girls Don’t Cry”: “Clarity, Peace, Serenity.”

So whatever happens, remember that big companies don’t cry either.

Have a great weekend and enjoy the video:

And exactly what does Miller–whose fund only holds Yahoo (YHOO) shares and not those of Microsoft (MSFT) too, as do many big shareholders of Yahoo–want?

Three guesses and the first two don’t count!

More money, of course, and no more thuggish threatening from Microsoft to drop the price.

In an interview with The Wall Street Journal’s most excellent Kevin Delaney published today, Miller (pictured here in a lovely dot-drawing) said Microsoft had “blundered” by threatening in a letter the software giant sent to Yahoo over the weekend to go hostile and perhaps even drop the price of the deal.

“The problem is Microsoft blundered with the letter this weekend,” Miller said to the Journal. “Telling the shareholders you’re going to take something away from them is not a way to get their support.”

If Microsoft does that, said Miller, Legg Mason–which owns about 7% of Yahoo–would back Yahoo’s efforts to stay independent and even buy more Yahoo shares, as they presumably dropped like a falling knife if Microsoft were to pull its bid.

The bigger question is whether anyone else will follow Miller, such as Capital Research & Management, which owns 11.6% of Yahoo (along with 5.6% of Microsoft), in publicly supporting Yahoo (or, in fact, Microsoft).

I doubt it, unless things got really ugly in a true proxy battle.

But BoomTown’s post last week that many significant shareholders indicated a preference to back the Microsoft deal was borne out by a Piper Jaffray (PJC) research note released yesterday, which the Journal cited, that surveyed 20 institutional Yahoo investors and concluded that “the majority suggest they favor the current deal to no deal.”

And, to me, the most interesting part of Delaney’s piece was a throwaway line at the bottom, in which Miller “dismissed the idea that Yahoo can find an alternative deal that would be palatable to shareholders.”

Since Yahoo is still ferreting away with execs from Time Warner (TWX) and its AOL unit–you know, that rat’s-nest of a deal in which AOL, Time Warner investment dollars and, oh, some old fishing rod of former exec Don Logan’s is thrown into Yahoo for a 20% stake, along with perhaps a dollop of Google (GOOG) involvement for added complexity–the lack of support from Miller for a more complicated scheme than the cleaner Microsoft deal is, to my mind, much more significant.

In any case, Miller’s support of Yahoo is absolutely no surprise given how close Miller has been to Yahoo over the years, especially due to the lavish attention and care Yahoo President Sue Decker, a former stock analyst, in fact, has given to Wall Street fund managers in general.

Still, as the major Yahoo-only shareholder, Miller has weight, although his higher-price plea is not a new tune.

In fact, when the bid was first revealed, Miller urged a higher price and cannot be too happy about the current value of the original unsolicited cash-and-stock offer of $31 a share. It is now worth about $29 due to a drop in the value of Microsoft’s shares.

And given that a higher price would surely bolster Legg’s performance, Miller reiterated his interest in more dollars from the Microsoft kitty in his interview with Delaney, even offering negotiating tips to Microsoft CEO Steve Ballmer.

“If Microsoft raises the offer, the pressure shifts very quickly to Yahoo to negotiate,” he said. “To me, bumping the number up a buck [from $31 a share], that would have a big impact psychologically on shareholders.”

Only one single dollar and shareholders get shiny and happy!? Message received, Steve Ballmer?

Please see this disclosure related to me and Google.