So the whole "social network" thing like MySpace and Facebook have sort of passed me by -- or at least I'm a very late adopter. I have a question. What if someone adds me as a friend on Facebook and I don't know who they are. Is it part of the etiquette to send them a note to the effect of "Who the hell are you?" Or, "Excuse me, but how do we know each other?" Or doesn't it even matter and I should just accept the invitation anyway?