My experience of Facebook has to date been only positive.
Every now and again I am contacted (or find) a person (usually a school friend) with whom I haven't been in touch with for ages, sometimes even decades. Living inside your own little bubble that is your life, one tends to forget that everyone else is also inhabiting their CO2 universe, merrily bubbling along the timeline of their existence.
People change. We all do but we often forget that others are still around, eking out their existence in totally different circumstances than your own. Yes, there are similarities as many have gone through university, got married and taken on the parental roles that we now all seem to wonder how we ever lived without.
Others have chosen different paths, but still we are all the same age as you, facing many of the same challenges that life seems to enjoy throwing your way. Before you realise it, it is twenty years since you saw so-and-so and that he or she didn't stay the same, as though frozen inside the slowly fading school photograph.
Just like you, they too have changed, but it often takes being in touch with them to realise that you are the not same spotty teenager you were when the two of you last spoke. It is unnerving, but strangely comforting.