Ning_logoBuild-your-own-social-network company Ning is joining the ranks of sites like Facebook and LinkedIn with the launch of an application platform for outside developers. While most social networking apps are targeted at users, Ning Apps are built for the social network creators themselves — you install them to improve your networking site.

For example, if you have an entertainment-related network, you could install the Ticketmaster app to sell tickets for relevant concerts and other events events. If you have a network designed for activism or nonprofit work, you could install the Tungle.me app to help schedule meetings. And if you have a sports-related network, you could install the NewsShare app to point members towards related news stories. Ning says it’s launching with more than 90 applications.

The Palo Alto, Calif. company announced the Ning Apps platform in May, but it was in private beta testing until today. I asked Ning about  TechCrunch’s report that the launch has been repeatedly delayed, and the comapny said, “We ran the private beta for a reason –- to get feedback from our Network Creators. We got a lot of great feedback during the beta that we took and incorporated into the product.”

What’s not clear yet is whether Ning Apps help the company build a business model that justifies its $750 million valuation. Back when the platform was announced, chief executive Gina Bianchini told us that Ning will take a share of the revenue earned by some apps, but that’s not the focus of the program. The apps are built using OpenSocial, the format first developed by Google to help applications work across different social networks.

Ning says 1.5 million social networks have been built with its technology, of which 250,000 are active. The company has raised a total of $119 million.