@ callingbird - "Why not just keep in touch with 'real' friends via email and telephone calls?"
I think Facebook is useful in its own right... you don't need to put much personal info down if you don't want to, or play any of the games (I don't). But I find it a useful bit-bucket to keep people in, especially those from the past; and email addresses, like real addresses, are often changing.
A pair of brother FB-friends lives on the other side of the world and I haven't seen them in 40 years, since we were neighbours in Beirut as kids*; we don't have the time or inclination to do a series of emails (apart from a flurry of messages when we re-established contact) but it is still good to know see what they up to, see pics of the family and in one case, see pics of a return visit to Beirut one of them made in 2008.
A former friend and colleague in England lives in the US, has 200+ "friends" and a new baby and is about to move into a new house; I haven't seen him in 15-20 years and it is interesting to know these things without forcing him to email 200+ people at this busy time on top of the genuinely close to him he must be phoning anyway.
But if I was in Indianapolis or New York or all those three people were over here, we could easily make contact and arrangements to meet through FB without a lot of hassle.