Linkedin is nice because it has a completely proffessional attitude.
Facebook sucks, it is just a reduction of something low enough in my opinion called mingeling.
I don't know what, of all those big social networks, is going to stay after the burst of buble 2.0, but I have a wild speculation saying that one effect of the new created tools (RSS, Web services, APIs and mashups) will bethe future death of the URL.
Therfore, One network that is beginning to grasp the new concept is, in my opinion, Imeem.