What, you mean you didn't read the T&C's, Del ? ;-)
As far as I understand it, Facebook (and the others in a similar way) stipulate that in using their services you agree to giving them pretty much total rights to use, distribute etc your images for their own purposes, but ultimately you still own copyright.
Your images do not, however, become 'public domain' after exposure on social networks - you own the intellectual property - so are still covered by the usual rules. How you defend against unauthorised use beyond the network after uploading images to FB etc gets difficult, though...(BTW 'Fair Use' is complex but basically revolves around public interest usage i.e., reporting, reviews, critique etc).
For a lot of snappers (and who isn't a 'photographer' these days?), this is not a problem: as Del points out giving over your pictures to the interweb can be a handy thing if you're happy to basically give up rights to the image but end-users (sites, bloggers) should be aware that the charity of all photographers should not be assumed.
Disclaimer: I'm not a lawyer, this is not legal advice - I just work in the industry.