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.

Last night, well-known Internet entrepreneur Marc Andreessen appeared on the “Charlie Rose” television interview show, talking about the digital sector and unveiling the news that he is creating a new venture fund.

I had heard rumblings about Andreessen’s funding efforts earlier this week, with sources I talked to nicknaming it “Project A.”

Actually, Andreessen said the new firm is called Andreessen Horowitz (zzzz), because he is doing it with longtime investing partner Ben Horowitz.

“For the first time in my life, I am crossing over into the dark side,” said Andreessen.

Although he gave few specific details about the fund, Andreessen essentially said he was simply putting a structure around his own active angel investing, which has included start-ups like Twitter, Digg, LinkedIn and many more.

His new effort will focus on early-stage investments, he said, noting that “our claim to fame is, we’ve actually, you know, by entrepreneurs for entrepreneurs, we’ve done it, we’ve been on that side of the table for a long time; we know what it’s like.”

Adding that he and Horowitz had made 36 investments over the last three years of up to $200,000, Andreessen said his new firm will make up to $1 million bets on start-ups.

Of course, for most of the interview, Rose zeroed in on hot topics like Facebook, the social-networking site on whose board Andreessen serves.

The voluble tech star did his job, talking about how Facebook could turn on the spigot and make all sorts of money anytime it wants and about the recent controversy around its Terms of Service kerfuffle.

He also talked about the Andreessen-backed Ning social network service, the Apple (AAPL) iPhone, Twitter, why the New York Times irks him, Google (GOOG), the Amazon (AMZN) Kindle e-book reader and gaming.

Ironically, the Netscape co-founder and his Xbox from Microsoft (MSFT)–eek–“have a very close personal relationship.”

But Andreessen also told Rose a little bit about the new fund he is raising money for, for example, while discussing the economic meltdown.

Talking about the fact that innovation will survive, Andreessen said: “Like with our new fund, if we fund a company today, we’re thinking about a return in seven to 10 years, so we can go through three or four or even five years of economic downturn, as long as, at some point, we come out the other end.”

Here is the video of Andreessen on the show (he starts to talk about the new fund in the interview at around 18:33 minutes, again at 46:55 minutes and at the very end):

Of course, the stock market had to come roaring back and it had to be extra sunny on the very day I was scheduled to have lunch with well-known Silicon Valley investor Ron Conway to talk about the worrisome state of the digital sector.

After all, it was Conway (pictured here), as well as Sequoia Capital, who sent out a stink bomb of an email last week to his start-ups to deliver a simple message: The Web 2.0 party is over.

Said Conway to his entrepreneurial troops as the market was crashing down and smacking tech companies hard in the process:

“Unfortunately history DOES repeat itself but I hope we can learn from history and prevent the turmoil from occurring again. The message is simple. Raising capital will be much more difficult now … the name of the game in this environment in some respects is survival–survival until conditions change.”

Survival!?! What ho? This from the man who is one of the more upbeat of Web investors! Who is one of the backers of Digg, Facebook, Twitter, Ning, Seesmic and 125 other Web 2.0 companies?

Ironically, our lunch yesterday was little more than a year after I had had another lunch with Conway, where I also made a video, and during which we had a lively discussion about a range of topics, including the venture business (better than ever!), innovation (better than better than ever!), monetizing video content on the Web (best of all!) and more.

Yesterday, the news was not so happy, of course, although Conway did try to explain in more detail exactly what he meant in his most recent email, which echoed one he had also sent out when the last Internet bubble was popping a half-dozen years ago.

And that is: You better have a year’s worth of cash and a revenue model or you’re toast.

Other than that, Mrs. Lincoln, he’s bullish!

Here’s the video (and below it, the one from June 2007, when things looked a little sunnier for Conway and Web 2.0):

Burda runs an annual digital conference in Munich called DLD (Digital, Life, Design), which a lot of U.S. tech types have gone to, including me, even though the media giant is best known for its old-line magazines about fashion, cooking and crafts.

But Burda has also been dipping into the digital sector a lot, especially via the tireless tech networking of the company’s Steffi Czerny and Marcel Reichart, who have established a high profile for the company among Silicon Valley denizens.

That was in evidence at Burda’s luncheon yesterday, which was crowded with a panoply of techies and hosted by Czerny, Maria Burda, the wife of CEO and owner Hubert Burda, and well-known Israeli investor Yossi Vardi.

Czerny has been taking Maria Burda (pictured here, she is also a famous German television star, better known as Maria Furtwängler) around the Valley on a learning tour this week to meet with companies like Ning, Mozilla, Facebook, and with venture capitalists. They are also here to attend Google’s Zeitgeist partners’ event, which starts today.

Here’s a video of yesterday’s luncheon, where I was asking everyone about the troubled economic situation and more.

It includes chats with Mullenweg and Seesmic’s Loic Le Meur, BillShrink’s Peter Pham, Facebook’s Brandee Barker and Randi Zuckerberg, Wired’s Steven Levy, Vardi and Burda and Czerny: