For some it's a medium to get to know people, for some it's just for friends. I sometimes accept people from uni whom I don't know well, if I see they're from my school and we have several mutual friends. But it's different.
I usually don't send requests to someone unless I've spoken to them personally several times. I find it a bit awkward if you meet someone and you both know you or the other person added the other and was ignored..
OP, it probably means she either doesn't recognize or doesn't consider you a friend (yet).