Facebook is this "social utility tool" that, in layman's terms, helps people find each other. Long-lost family members, classmates, university pals, and best friends can catch up -- without the hassle of emailing or calling. After you hook up, you don't have to work hard keep the connection alive; you just send a quick one-liner or check out your friend's profile to keep in touch.
I like it. It sucks too much of my time (I should be writing!), but reconnecting with old high school friends and long-lost family members is good for my health.
Even touching base with "enemies" or people you've hurt or disappointed can be good. You can send an apology or wishes for a good life, you can eat some humble pie and try to make amends. Even if you're rebuffed or ignored, at least you can go to bed knowing that you tried to make up.
Finding your old friends and/or facing your demons can be the best thing you can do for your mental health.