Ever tried mail and the ocassional phone call or meeting?
Do you really have to keep in touch with people seeing what they did on their vacations, bs inspirational quotes and baby photos?
You may mock, but keeping involved in the minutiae of each others' lives does enable the maintenance of friendships over longer distance and time spans. Minutiae that we wouldn't necessarily make the effort to email or chat about.
As I get older my friends (and myself) have dispersed all over the globe, and even the ones not so far off have less time to give (and possibly less inclination) to spend time just hanging out.
That said, it doesn't have to be FB, or a centralised service at all.