I also vote "not rude." I opened a Facebook account a while back and only "friended" my brother. Then in a fit of energy I recently friended my sister in law, bro in law, and a cousin, because while I don't actually post anything on my account (and I keep it set to pretty much the most private settings), it's nice to keep up with others. Well, that set forth a bunch of friend requests, mostly from other cousins. I went ahead and accepted them, until I got a friend request from a veeeerrry distant relative that I have met exactly once. I'm sure she couldn't pick me out of a lineup. Likely she sent the friend request just because FB suggested me to her as we have mutual cousin friends. So I made the decision that I would friend cousins within my generation, but anyone else gets ignored. Thus far I've ignored the distant relative, and another cousin from an older generation with whom I also never have any contact anyway. I don't think it's rude, and I doubt either of them is crying in their Cheerios that I haven't friended them.
This makes, I think, 9 "friends" that I have on FB. And I can't keep up with them!