Facebook isn't like a phone call, or an e-mail or text to a single recipient. It's more like a party line: Everyone chats with everyone else, which means you'd better tread lightly in what you say. Don't complain about your insufferable boss, your ineffectual coworkers or your demanding client unless you're prepared for everyone to know exactly what you said.
Practice a bit of self-policing. Watch your word choices, your subject matter and the time stamps of your messages (lest someone notice how many postings you're making during work hours). You might feel like you're watering down your feelings, but discretion is an essential work skill and Facebook is the place to practice it. After all, you don't wander around your office proclaiming your every thought, feeling and exploit at the top of your lungs, do you?
Remember, too, that you don't have much control over something once you've written it; there's little to prevent someone from reposting it, and then you'll be left with the unfortunate choice of lying and denying -- or owning up and facing the music. Only post messages you'd be willing to have anyone read. This amounts to a good general rule for the internet, by the way: You should never consider anything you post online -- or send by e-mail, for that matter -- to be secret or protected.