Here's a story you've probably already heard at least once or twice in your life: Ben was a good employee. Always on time, always polite, always good with people. One day, his boss was pissed and yelled at him. When he got home, Ben went online and called his boss a f---ing douche on Facebook. His boss saw it and fired him the next day.
Sucks to be him, right? Seems pretty obvious that he shouldn't have done what he did in public. In fact, he could've yelled that his boss was douche (in the comforts of his own home, that is), and he wouldn't get fired for that.
Which brings us to the first thing people should never share on social networks...
Rants about your boss. Chances are, they'll see it. If they won't, then any competitive sneaks who are on your friends' list might do the tattling to get you in trouble.
Complains about your wife/girlfriend/lover. The reason why is pretty obvious. Don't complain about your girlfriend on your wall. Don't complain about your wife. Don't complain about lovers. And especially don't complain about all three of them--at the same time.
Pictures from last night's wild party. Like it or not, prospective bosses (and even dates) check people's online profiles first to see who and what they're like. If they see pictures of you dancing on tables or chugging down beers like there's no tomorrow, then you probably won't get the job (or the girl.)
Anything else that's supposed to be private. Your bank info. (Remember the debit card fiasco on Twitter?) Your phone number. Your license plates. Your home address. Your current location. And the list goes on... You get the picture.