I think one problem with "social networking" sites is that there are so many of them, and there is no interoperability. It's somewhat like the old Instant Messenger services, some of which are finally getting around to offering cross-service messaging. There are workarounds for that in the form of third-party clients like GAIM/Pidgin and meebo. Not sure if any exist for the social networking stuff, although meebo has absorbed some of the messaging components.
I use Facebook; I ditched MySpace a long time ago due to spam. Facebook is headed in that direction, although it's not quite as obnoxious with the page layouts and huge ad blocks. I do have a LinkedIn account, but have only 3-4 "connections".