A lot of people in the social networking space talk about the need for only one social network (Metcalf's Law), or that we should all have one user profile (OpenID). They talk about how this is limiting social networking because if a site gets too big and powerful because of the network effect, then it loses what originally brought people there, becomes too commercial, etc. This of course gives rise to lots of "mini" social networks (Ning hosts 160,000 of these!).
I think there is another phase to social networking that is coming - social applications, not social networks. Facebook unlocked this first with the open API that allows developers to develop "social apps" that cooperate with the social network.
The next limit to bust thru is to federate the social networks more. Open Social's promise is to do that - of course that is still very new and the social networks supporting it are still developing it. So the developer community building social applications is a tiny fraction of the Facebook community.
The real eventual promise is to enable social applications to be built and run anywhere and to interoperate with multiple social networks. This would allow the power of the Internet to truly take over. A federation of social applications working with a federation of social networks. I guess that was the promise of Web 2.0 (or Web Services or E-Speak for those old enough to remember).