It can be rather bizarre, that certain people add you as a friend on Facebook. I know that for the sake of boosting my friend count, I have added classmates from high school (and even elementary school) with whom I would probably not associate much in real life, even if they lived nearby--we just don't have that much in common. Socializing at a class reunion wouldn't be too bad. But other than that...
Perhaps they are just doing the same.
Anyway, someone I've known since 5th grade but haven't seen since 8th grade added me. I don't know why, since we don't have any friends in common (that is, no one from elementary school or junior high), so I have no idea how she came across my name on Facebook, or what spurred her to look me up.
It's not like we got along that well. Actually, during middle school I didn't like her that much ('like' in the 'innocent' way) since she acted a bit snobby (and materialistic), and considered herself part of the popular crowd.
We'll see if she responds to my greeting.