I don't see how you could sue them for slander or oral defamation (which, btw, are the same thing) because yahoo! answers has no voice with which to speak...
As far as libel goes, that depends...certainly you can't sue yahoo! answers for anything that someone else said about you. If Yahoo told someone something about you that was untrue and caused you monetary loss, then yes...you can sue them....otherwise, you're outta luck.

