most of the time, i just kind of go, "oh, well okay," and then unfriend that person because i figure if they don't want to read my stuff they probably don't want me reading theirs either.
every once in a while though, someone that i thought i was getting on well with will unfriend me, and i'll admit it -- i take it a little personally. like i'm not good enough or something.
i rarely delete anyone -- even users that haven't posted anything in years.
i guess i like to think that, if they ever come back, then i can go, "SEE? I NEVER LEFT YOU."