Facebook is good for keeping in contact with people who you don't see or talk to that much anymore.
I have a private account where I only add people who are actually friends, or people who engage in my special interest, so that's around 50 people.
I added very few people in my Uni course (only added the ones who are loners and don't have a clique) coz if I add 1 of the regular NTs and not add the others, the others will think that I'm snubbing them on Facebook which I don't want. There's like an "add/accept everybody on Facebook" phenomenon in my course.
The reason why I don't add the people in my course apart from the loners is coz the majority of the NTs in my course joke and banter with each other on Facebook which I find banal and boring, and yet when I try to join in the "fun", they think what I say is "disturbing" and I just killed their thread.
I tried falling to their level and I failed, I don't like trying to be something which I'm not.
I feel they're applying a double standard and I don't think they're good friends if the only reason they'll get close to you is by you being all funny and stuff (even if you have nothing else in common).
They don't really care about me, and I don't care about them, so I'll save myself a lot of pain and time by not adding any of those people...