Social networks are hot. People like to connect, and marketers could potentially make a killing with access to your social graph. Facebook alone is currently valued at $6.5 billion. Even Google’s getting in on the action through Google Friend Connect.

Imagine, then, the value of a service to create your own social networks. Ning, the hosted social network solution from browser inventor Marc Andreessen, just raised $15 million from Lightspeed Venture Partners. This latest investment round values the startup at — get this — $750 million. That’s pretty good for a freemium service without its own advertising platform, especially since only three percent of its network creators actually pay for ad-free service.

Free self-hosted social networking scripts abound on the Internet, but the average community leader has neither the time nor the patience to install and maintain such scripts. Ning could do for social networks what Blogger once did for blogs — idiotproof them for the masses.