Facebook is more dangerous than people realize. It's kind of ironic because you can write something on the net and theoretically speaking anyone can find it, but they won't in all the other stuff. But on facebook people have a direct line to these things and so these stories about people getting in trouble over a remark, a picture.. well, they resemble more closely an equivalent event in a real life social situation.
If you stand up at work and say your job is boring, well that presents a certain problem. What about all the other people who are doing the same thing, it doesn't help the company if they all leave. So they don't want you to say things like that. It's a fine balance.