It seems to happen most if you use hotmail - the two accounts (FB & HM) link together and if you ask for a friend search then it automatically trawls all your hotmail contacts. I've never done this myself but I can see how easy it is (and how tempting if you are an FB fanatic.) As Nick says, this was discussed recently on a thread about FB - I think it had the title "Like or Unlike" (sic.)
You are right of course, Leo, s/he wasn't.
"I couldn't sleep very well last night. Some noisy b*ggers going around in automobiles kept me awake." Ken Miles