Very true. But at least in most forums and art communities they try and do something about art theft. Facebook's only really given you two options.
1. Let the thief get away with it
2. Give them your contact details which will be passed onto the art thief (and then there's no guarentee he'll even listen to them anyway).
Then of course there's the privacy settings. For all I know, somebody could have stolen my artwork and uploaded on their page without my permission. But because of the privacy setting on Facebook I'll never know.