Facebook keeps my large, globally-spread-out family in contact in a cohesive way. We don't have to click through annoying pop-ups and registration forms just to view pictures. We don't have to remember e-mail addresses or remind family members that it's .US not .COM, for certian addresses.
It has also been useful for my job as a reporter, since I can get in touch with certian people without climbing through hoops. Tracking down local Marines, sailors or soldiers is easy and has proven instrumental in some stories I've had to deal with. What would have taken a couple days of voicemail hell is as simple as remembering that I went to school with someone and I can send a message that way.
If you hate Facebook, it's probably because you think it's an excuse for a social life. Well it isn't.