I ended up on LinkedIn because so many ex-colleagues were on there inviting me. I have found it to be more useful than annoying (which is how i rate these things.)
I ended up on FaceBook when my wife joined it on the urging of my college age children. So far, that has been mildly amusing, but it is not more useful than annoying. But this is just my opinion, don't take it as a definitive review.
On the other hand, all of these things are popular and successful sites, and being on them helps me figure out where the Internet is going. I need to do this from time to time, because I own an Internet company.