I think it's useful to have a friend's policy for precisely these reasons. I don't generally get upset if someone defriends me, unless I know them in RL. And when I do know someone in RL, but feel like I need to make a break, I usually send a quiet note to that effect, but privately.

Strangely though, no one's defriended me yet--even people I've taken off my list, though there's definitely been people who I've friended who haven't friended me back.