Because Facebook does not have extensive copyright preferences, artists who share their work on this website are not guaranteed what will happen to their art down the line. As more people choose to use Facebook as a media sharing website, Facebook will have to re-evaluate what rights they grant to you as a user.
Do you think it's necessary to define all that is public or private on the Internet? Should this distinction determine whether or not we may quote these online works (ie. Facebook, Twitter, e-mail content)?