Not quite. Yes, network effects are important, but if that were the only important thing, then Facebook would be the only social media site. The opposing dynamic is that it's not hard to be on multiple sites at the same time. So if there's a site that meets my definition of "better" and has critical mass among the people I'd like to connect with it, it can compete. Each new generation gets its own social network because it has no interest in connecting with the squares on Facebook.