LinkedIn isn't that relevant to me; I made a profile to see what it did. Myspace is dreadful, but you pretty much can't not have one anymore. FriendsReunited is also more or less compulsory. Of all of them, Facebook looks to have a lot of potential, but it's so jolly hard to track people down via it. I know I must know loads of people who have a profile there, but I can't find them.