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 is still in an Elmer’s glue coma from making 252 handmade cards with 33 pounds of glitter, which my sons’ school insists on instead of the store-bought kind from Walgreens that I prefer.)

In any case, big, sloppy hugs all around to the readers of All Things Digital–except for those who know just who they are.

But to make up, please don’t think sending a romantic Twitter to us or one of those dumb digital virtual gifts of silly-looking little animals and baked goods on Facebook is going to cut it.

That’s right, you need to send a whole email to show that you really care! And maybe an iTunes gift certificate for Jill Sobule’s fabulous new “California Years” record too (I am wearing out my advanced copy, but it is officially out in April, including the apt song, “Bloody Valentine”).

Also a Tesla.

After all, love is bling.