I get friend requests from people I bearly know. People who have just added me because they've added every familliar name on their "friends'" lists.
I have people on my friend list who have over 400 friends. nobody really has that many friends.
So I have come to the conclusion that facebook has more to do with people's need to be popular, and to be seen as being popular, as all their friends can see when someone else wants to be their friend "x and y are now friends".