This has cost $119 million and counting.

Ning is a pretty innovative company that allows users to create their own social networks and monetize them through a number of ways, including Google ads, microtransactions and premium revenue.

They recently closed an additional $15 million of funding (bringing their total to $119 million) at a valuation that pegs them at $750 million.

That’s Bebo.com-like money, a social network bought last year by AOL for the same amount.

BoomTown has some details:

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.

Not all the metrics are great, though:

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.

The additional funding will be used to acquire more social tools for its users, increase its userbase through marketing, and adding new team members to help with its monetization.

We’re not sold on the model; frankly, it seems a little more like GeoCities than anything. But the funding is substantial, and proves the saturation point isn’t quite there for social networks. At least where investors are concerned.

For now.

[BoomTown has the scoop]