As Ning's U.S. Audience Flattens, It Raises Another $15 Million.

Do-it-yourself social network Ning has added another $15 million to its coffers from LightSpeed Venture Partners, the company has confirmed to us. This brings the total capital raised to $119 million. Its other investors include Allen & Co., Legg Mason, chairman and co-founder Marc Andreessen, and Reid Hoffman.

Ning offers a counterpoint to the uniformity of Facebook, allowing anyone to create their own social network customized to their particular interest or social group. Earlier this year, Ning passed one million social networks created (it is now up to 1.3 million), but the key is how many of those are active and how many people they attract. In the U.S., unique visitors actually declined 10 percent from May, 2009 to June, 2009, according to comScore. Ning had 5.1 million visitors in the U.S. in June (its worldwide audience is about three times as large).

The company attributes the decline to “some downtime in June as we expand and optimize our infrastructure to support the growth that we are expecting in the next 12 months.” Ning says it is adding 4,000 new Ning Networks every day and one million registered users every 15 days.

One month hardly makes a trend, but Ning’s fragmented approach to social networks has yet to catch on in the way that Facebook’s monolithic strategy has in terms of activity or pure audience reach across the network. Back in April 2008, Ning had a half-billion dollar valuation, and now it’s supposedly $750 million. That is getting close to the inflated level Bebo was able to sell itself for, which we now know was too high. And Bebo still has more people using its product. (Even in the U.S., it had 8.7 million unique visitors in June).

Ning has been getting its act together, though. Back in December, it expelled adult networks from Ning because they aren’t advertiser-friendly. And that worked out well for them.

More recently, Ning has been working hard to make its social networks compatible with OpenSocial apps. That effort is going much slower than expected. A public launch was delayed last month because of performance issues with the Ning Apps platform, says one developer who is part of the program.

Maybe the new cash will help speed things along.