The downside of not participating professionally in social media sites is that increasingly photo editors and art buyers are looking to them for inspiration, and to find new styles and new photographers. And with the current generation of 20-somethings entering the photo industry, who have spent all their adult lives participating in MySpace, Facebook, Twitter and Flickr, this will only become even more prevalent.
So that you know what's what, the ASMP, America's version of our own AoP, have put together a web page that explains to you in plain language the T&Cs of the main social media sites, letting you know what they, and other users, can and cannot do with your uploaded photos and videos.
Something everyone should read IMHO. Go here for the details.