It's a way to stay in touch with people without having to actually talk to them.
It's a way for people, who annoy you, to feel like they are staying in touch with you. People like your crazy aunt who will call or come for an unannounced visit. Instead you let them be your facebook friend, then you post a couple of pictures every 5-6 months, and they don't call you or come buy. Yet, they still feel like they know what's going on with you.
Basically Facebook, when used properly, is a diversion tool to keep annoying friends/family/co-workers out of your hair