Social networking has been the hottest area after the dot.com burst. User base is still a critical factor and far more sticky than before. It’s the connections among these users that differentiate social networking from other types of Internet services.
A connection is formed from one user to another. They cannot be on two different web sites, even though theoretically they can. Technically we can define protocols to link users, even groups, together from different sites. But it is not efficient and may not be fast enough to sync up states and discover new connections. Even more issues on business side, not to mention privacy policies.