During my lifetime I've lived on 6 cities in 3 different countries. I've got friends (some of them, became very good friends) and people I know from all these places.
Facebook allows me to stay in contact with them, and if I want, to peek at where are they know (and what are they doing). For me, it works as an "information push" platform, where sometimes I'll see an update from an old time friend who I forgot about, and I would use that moment to say hello and congratulate him or give kudos for whatever he is doing (or just say 'haha' at whatever funny picture he shared).
Before that, I had to remember to send an email to every person I knew... and ask them specifically what where they doing.
Sure, some people might find no sense in maintaining those kind of relations (staying in touch with people who you do not actively talk to). But I think we never know when we might be able to get together again, and having this point of contact is good.