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.

Yet another Web 2.0 wunderkind got itself more shelter from the economic storm–widget maker RockYou announced today that it has nabbed a $17 million investment from two Asian firms, SoftBank Group and SK Telecom Ventures.

The investment will be added to $35 million from the Redwood City, Calif.-based start-up’s C round in June. Overall, RockYou has raised a total of $67 million from investors, including DCM, Lightspeed Venture Partners, Partech International and Sequoia Capital.

It is unclear if the company’s valuation is at the previous $400 million level or not.

In a video interview with BoomTown on Friday (see below), the company’s co-founders, CEO Lance Tokuda and CTO Jia Shen, said the money would be used to expand into the Asia-Pacific market, including onto Xiaonei, one of China’s largest social-networking sites.

As part of the investment, SoftBank–which has major Web investments all over Asia–and RockYou, the company said in a press release, “will also set up a new joint venture company that will develop widget and application products and services for use on PCs and mobile devices in the Asian market, in particular the Japanese, Korean, Russian, and Chinese markets.”

Well, that’s a horse of a different color, what with most Web 2.0 outfits pulling in their horns of late.

Indeed, RockYou said it would also use the money to open offices in New York, Los Angeles and Detroit and look at making some opportunistic acquisitions, as well as adding to its advertising sales force and developing more brand and vertical channels.

The company’s NYC sales office, previously a one-man shop, is expanding, adds MediaMemo’s Peter Kafka. It hired two people since August and still has one position open and plenty of applicants, says sales boss Paul van de Kamp.

Why all the frenetic activity from RockYou, which says it had over 100 million monthly uniques with over eight billion page views from its popular third-party applications like Super Wall?

Well, a few good reasons, such as: the need to keep up with its key competitor, Slide, which raised its own huge war chest; to find new audiences away from the two top social-networking site, Facebook and MySpace, from which it gets most of its traffic; to improve its products, in order to get better ad rates; and, most of all, to weather the current econalypse in the ad sector.

While RockYou’s execs believe the ad market for widgets and social networking is on the rise in comparison to other kinds of media, it will still be a glum outlook for everyone for a while.

Tokuda and Shen talk about that, as well as the investment, in the video here, and below that is a video I did a year ago with the pair:

RockYou, widget maker, is the latest example of a sane valuation heartbreaker, as it is undertaking efforts to secure an investment from mainstream financing firms that would value the company at between $300 million and $400 million.

First reported by Valleywag last night, the start-up, said one source, “is being squired around Wall Street” by investment behemoth Morgan Stanley (MS), in search of the same kind of deal its rival Slide got in January.

BoomTown broke the news of that deal, which nabbed Slide $50 million and a $550 million valuation with investments from blue-chip investors T. Rowe Price (TROW) and Fidelity.

Thus, RockYou’s motto: Anything Slide can do, we can do slightly smaller!

And, indeed, not to be SuperPoked by Slide CEO and Founder Max Levchin, sources said RockYou Co-Founders Jai Shen (also CTO) and Lance Tokuda (CEO) were quickly on the march for their own payday.

It is, in fact, a quest that a lot of Web 2.0 companies seem to be on, since the sector’s fearless leader–Facebook–got its $240 million and $15 billion valuation from Microsoft (MSFT) last year.

All of this frantic funding activity is, of course, this bubble’s version of going public–grab big cash investments from investment firms and hedge funds, desperate for a good bet on the sector, without the pain of public scrutiny of questionable business prospects that did in Web 1.0 shooting stars.

It’s that or get bought for an ungodly sum by equally desperate Web 1.0 companies (See: AOL+Bebo).

Sources close to RockYou, which has had acquisition feelers put out to it from larger companies in the past, said the company has had several strong offers of funding, but it is trying to select the right partners for the latest round of funding.

“We want our investors to be strategic and helpful to the company,” said one person close to RockYou.

RockYou has so far been funded by Sequoia Capital, Lightspeed Venture Partners and Partech International.

(Interestingly, Sequoia backs another instant messaging and chat widget maker, Meebo, which is reportedly seeking a $250 million valuation, which I posted about here yesterday).

To be fair, makers of highly distributed third-party apps like RockYou are garnering immense traffic and their widgets are syndicated everywhere. RockYou’s Super Wall, which lets you turbocharge your digital wall, for example, is one of the most popular on Facebook.

Other RockYou apps include: X Me, a communications tool that allows you to “Hug Her, Slap Him, Tickle Them!”; and Likeness, where you can “compare yourself with friends and movie stars like Angelina Jolie, Jessica Alba, Keira Knightley and many more.”

The company has been trying to monetize all this traffic and popularity and distribution, as well as knowledge of user behavior, by offering advertisers new forms of engagement.

But the jury is still out on these interesting but unproven efforts by all the social-networking players.

In any case, the money is apparently still flowing into these start-ups, taking a chance on them being the next big media play.

Here are two videos I made when I visited RockYou’s offices in San Mateo, Calif., last October, after I had called the widget market juvenile and faddish.

The first is my tour of the office, where I was playfully accosted by an infant–oops, a RockYou engineer–in a suit. The second is my interview with Shen and Tokuda.

Yesterday, a source told me that Flixster, the fast-growing social network for movie lovers, was back in talks to be bought by Barry Diller’s InterActiveCorp.

Those IAC-Flixster rumors flew about a month ago and were entirely true.

But no deal, it seems. IAC has apparently found the price too high, according to other sources. But, said these sources, others are still in the game.

So if an acquisition deal does happen, two intelligent guesses to the possible winner for the company with 39.5 million user home pages, more than 1 billion user-generated movie ratings and a sassy motto of “Stop Watching Bad Movies”?

Blockbuster or News Corp.

(Sources said Viacom’s MTV unit gave Flixster a look too, but also thought the price too high.)

The former because the video-rental chain has been mightily trying to break into all kinds of Web markets to give it a place to sell online. Blockbuster has been struggling of late, because of its costly battles with Web sites like Netflix and the growth of video-on-demand and movie downloads via the Internet and cable. Its online subscribership is only 3.1 million.

And the latter, because such a site tucks in nicely with some of the media giant’s properties like MySpace and Rotten Tomatoes, as well as its numerous movie and television arms. (FYI, News Corp. is in the process of acquiring Dow Jones, which is the owner of this site.)

The price? I will wager $150 million in cash that had been previously bandied about, although IAC’s offer was likely much lower than that.

Pricey, yes, since Flixster only has 17 employees and was created less than two years ago. It received $2 million in funding in February from Lightspeed Venture Partners.

But its vibrant Web site is booming and is now one of the largest movie sites on the Internet. Flixster is also one of the more popular widgets on Facebook, with more than 800,000 active daily users.

While Amazon-owned IMDb (Internet Movie Database) is larger in terms of traffic, Flixster is growing much faster.

On the site, users have profiles typical of most general social networks, with a laser-focus on movies, actors, directors and movie quizzes and news. Flixster also has video content, both professional and user-generated and relies heavily on user reviews.

So I recently ventured into the heart of the empire of toddler developers with a visit to the San Mateo, Calif., HQ of RockYou, the super-popular maker of third-party apps on hot social-networking sites like Facebook and MySpace.

I have been on a bit of a grumpy tear of late about the juvenile nature of these widgets, whose use has taken off explosively, as the sites they live on have grown.

I have felt that most of them have been a bit silly, useless and faddish, rather than long-lasting and relevant.

RockYou’s apps, for example, include: Super Wall, with 1.26 million active daily users on Facebook, which allows you to turbocharge your basic posting wall; X Me, a communications app with 706,000 Facebook users, which allows you to “Hug Her, Slap Him, Tickle Them!”; and Likeness, where you can “compare yourself with friends and movie stars like Angelina Jolie, Jessica Alba, Keira Knightley and many more,” which has 611,000 active Facebook users.

Like another widget maker, Slide (I did a post and video on Slide here, as well as a three-part interview with founder Max Levchin too), the start-up has big VC backing. In RockYou’s case, it is funded by Sequoia Capital, Lightspeed Venture Partners and Partech International.

Of course, there are the rumors of big-money buyouts and even IPOs for these developers.

I am not so sure this is a good thing, but I do also believe there is something important going on with companies like RockYou, which could become akin to the major software makers of the past era. If, of course, they grow up a bit first.

Here’s my video of a visit to their office (and here is an accompanying interview with its co-founders Lance Tokuda and Jia Shen), where one employee jokingly played dress-up just like an adult, sporting a suit and tie just for me.

Oh, those crazy kids!