They don't.
A question about the rating system

Hi PythagoreanA,
I think you'll find several (many) posts on this topic already, so you're probably better off reading those than re-hashing the subject all over again. Here are a couple of relevant posts, but there are many others too:
http://www.chess.com/forum/view/general/chesscom-rating-vs-fide-rating3
http://www.chess.com/forum/view/general/chesscom-rating-versus-uscf-rating
I've never played chess seriously outside of this website, and I was wondering how our ratings here would come out as FIDE/USCF ratings.
I.E. If my rating here is x, what would it be if I was a part of the FIDE/USCF/BCF/etc.