And here is the wikipedia page on Social Networking sites- over 200 and counting - http://en.wikipedia.org/wiki/List_of_social_networking_websites. It's interesting because it lists the number of subscribers to each site. As you can see a lot of silos.
What we need is a Meebo for social networking sites. Meebo allows users to IM across several IM systems - we need the same for Social networking sites...