Esther Dyson said this at SuperNova:

Most of these social networking tools won’t survive as businesses. Almost any function will be part of some other product. They may survive as functions within business, but they might not survive as such. Things like LinkedIn go way beyond the Dunbar number. It’s akin to friend inflation. You get a kind of fake niceness. They need to lose their buzz and go away, and people need to focus on their true networks and circles of friends.

It explains why these services are super-popular in the Bay Area. This part of the world lacks the solidity of rest of the US, and the friendships here are well … as Dyson said… full of fake niceness.