Facebook "Friends"?

Does anyone REALLY know 300+ people? I remember when I first got Facebook I didn't understand how people could be friends with so many people, and I still don't. Sure they go to the same school as you, but you never talk to them...at all. They are in your class but you never say hi. Everytime someone adds me, I always hesitate to add them, my friend always makes fun of me for it. She says "Just add them, as long as you know who they are then its fine". I wouldn't want them to have access to my information. It may seem harmless but you can't be too careful. I don't really add people unless I have talked to them at least twice and who I know I will talk to in the future. I sometimes add people who are old friends, but at the same time I don't even talk to them on Facebook.. I think its funny when I deny people's friend requests because sometimes they re-add me. Do they want to be my "friend" that badly? I remember when I first had my 100th friend (which wasn't too long ago)... for some reason I didn't want 100 yet, I kind of wanted to delete people who I don't talk to anymore lol . I don't think its that bad, I just like to keep my friends close I guess..