Facebook Dilemma

What do you do when someone you don't think you know friends you on Facebook with a cheerful personal but not that personal note?  I mean, if you click on the link to see their profile, you end up friends with them, but then what if you don't actually know them?  Clearly this does not bother those who have 579 friends, but I don't think I want to be Facebook friends with people I don't know.  I