Knowledge@Wharton has a fresh article on social networking business models. Ross Mayfields is  quoted a couple of times, its a nice piece. Particularly this simple point about what's new:

According to Wharton marketing professor Peter Fader, networking services may succeed where a multitude of dot-coms have failed for a number of compelling reasons. One is that the Internet has become a part of just about everyone's daily life, including in some cases their social lives. The second is that in an era in which the Internet has also become a tool for communications, the networking services indeed have something tangible to offer – the ability to connect with people who may be important to you. In fact, says Fader, he has personally experienced the value of networking services as a member of LinkedIn, one of the hotter networking services around.