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.

As BoomTown expected, the popularity of the chaste-yet-lustful vampire movie for screaming teen girls everywhere, “Twilight,” has inspired more online video spoofs than you can shake a braided rope of garlic at.

Rabid fans of Stephenie Meyer’s book series flew into theaters all weekend, adding up to a $70.5 million U.S. box office tally so far.

Thus, we are in for more online videos in praise and in mockery of Edward and Bella than perhaps even Alaska Governor Sarah Palin engendered.

Here’s one of the oh-so-serious “Twilight” trailers, followed by one of the better spoofs–by Evil Iguana Productions–below it:

Yesterday, BoomTown opined that Microsoft was nearing a decision on who would become the head of its digital efforts.

And, according to several sources and some puzzling by me–if an agreement can be reached–I think that Microsoft (MSFT) CEO Steve Ballmer’s top choice is former Yahoo tech star Qi Lu.

While this is by no means a done deal, Lu is just the kind of top tech exec that Ballmer and Microsoft would warm to over a more media-centric choice like former Yahoo COO Dan Rosensweig or former AOL head Jon Miller.

Lu was EVP of engineering for the Search and Advertising Technology Group at Yahoo (YHOO), where he ran all development initiatives for its search and monetization platforms. He was at Yahoo for a decade.

If Ballmer manages to pull off the hire of Lu–on the heels of already grabbing another top Yahoo search exec, Sean Suchter, which I reported on yesterday–the aggressive exec could almost be bypassing a Yahoo search partnership he has long sought by sucking the talent right out of the place instead.

Ballmer is like Edward in “Twilight,” attracting top-notch search execs to Microsoft’s Redmond HQ, as if they were geek versions of Bella.

Lu would be a different choice for the post than many had expected, with a much more technical background than one in online media or advertising sales.

But since all of Microsoft’s future rests on winning in the search and search advertising space and trying to catch up with its archrival Google (GOOG) from way back in the race, Lu is also well suited for the position.

If Lu takes the job, he will be the boss of three strong digital execs at Microsoft: Satya Nadella, the SVP who heads engineering for Microsoft’s search, portal and advertising platform group; Yusuf Mehdi, whose online services portfolio includes marketing, online audience business development and product management for MSN and the search properties; and Brian McAndrews, the SVP for the advertiser and publisher solutions group.

Lu is known as as solid manager, but he is also called a very nice man and unusually humble for a tech star by many, which could be a good influence on Microsoft.

Before Yahoo, Lu was on the staff of the IBM Almaden Research Center, and worked at both Carnegie Mellon University and Fudan University in China (he also got degrees from both places).

And, in the kind of cred Microsoft likes, Lu holds 20 U.S. patents.

He left Yahoo after becoming dissatisfied with all the turmoil there, quitting in June, without another job lined up.

Since he left Yahoo, there have been rumors that he might be headed to Microsoft, but not in such a prominent job.

There has also been speculation that Lu would take a position at Facebook or even return to China for a tech job.

The well-respected Lu certainly has a multitude of choices, but the chance to lead money-laden Microsoft’s digital efforts–as it suits up for battle with Google–is compelling.

BoomTown has been poking around to try to figure out who Ballmer would choose for the digital head, ever since the man who used to be in charge, Kevin Johnson, quit in July, after the software giant’s takeover bid to buy Yahoo failed.

Several people close to the situation say Microsoft’s Ballmer has been keeping the deliberations close to the vest–perhaps because so many of those he has targeted have declined to consider the job.

But this week, many sources both inside and outside the company have told me that Ballmer is close to announcing his choice.

Annoyingly, one source has decided to play a digital version of “The Da Vinci Code” with me, dribbling out clues–more technical than media, very well liked in Silicon Valley, humble–about the candidate, which he wanted me to solve as if I were Robert Langdon and on the hunt for the progeny of Jesus.

Well, my solution is in: Microsoft’s most promising digital Holy Grail is Lu.

On a related note, bizarrely, the day after this column broke the story about Lu’s leaving Yahoo, I caught him by accident in the background of a video I was doing at a Harvard Business School event honoring Facebook’s Mark Zuckerberg.

You can see him at 4:14 minutes in the video, laughing at me, as I bother Greylock Partners VC David Sze and make a bad pun related to former Yahoo exec Jeff Weiner’s departure from Yahoo.

Here’s that video:

Please see this disclosure related to me and Google.