I think most people consider what they post on social media sites to be for their friends, not the public. Of course, under an American moral definition of privacy, if you tell something to one person ever, it's as public as if you bought billboards across the country. And if you're tricked into releasing something to the public, say by a company which fails to properly advise you when you attempt to share things with your friends but actually publish them to the wider Internet, you have no recourse, legal or moral.