Yeah, danah points out that even though there isn't an economic reason for the segregation, it's happening anyway.
Wouldn't the most likely explanation simply be that the connections on social networks tend to mirror connections in real life? I think there was a study posted to HN recently that basically said instead of connecting to everyone and their dog, connections tend to stay local.
I am also not friends with everyone on Facebook, so I guess a lot of opportunities are denied to me (because Chad doesn't tell me about the job offers either). It's a cruel world.