The social networks all have a “Terms of Service” that you have to agree with. Read carefully.
I just read on another website that Facebook changed their TOS. It did say that they basically can do anything with your content, including relicensing it, as long as you are a member of the site. They just took out the language that keeps them from thinking they own it in perpetuity.
The link to the article is here: http://bnsn.com/YfE/
Almost all of these sites have this sort of thing in their TOS. A lot of us use these sites for networking and as a useful tool for promotion. With Facebook’s mobile app, I can access images I have their to show friends or potential clients, get phone numbers if I somehow don’t have them in my phone, etc. But they have the same problem that a lot of the photo contests have. Lawyers that want to protect them to the detriment of the photographer’s rights. This is a bit of a hole in the copyright system that may need to be looked into.
Be very careful what content you upload if the site has TOS that strip your rights or grant them rights they shouldn’t have.
Update: After the furor created in all the news, Facebook has reverted to the language of the old TOS. It’s still a little grabby in the rights department, but it’s much better. They claim their intentions are nothing but good. I’ll almost buy that, and with a grain of salt after some of the other trouble they’ve had. Still, the blogosphere spoke and they listened.