If you don’t want your employer seeing your Facebook updates:
1. Don’t friend any of your coworkers (or, at least, your superiors) on Facebook.
2. Go to http://www.reclaimprivacy.org/ and run their privacy scan on your Facebook account. Lock down your account per the site’s recommendations to ensure that your account is not visible to anybody who is not your friend on Facebook, including your employer.
3. Don’t post anything on your Facebook page which could be considered a “career limiting move”. You never know who might see it and say something about it.
4. Delete your Facebook account. Nobody can fire you fire what you say on Facebook if you’re not on Facebook to begin with.
Just think of Facebook as that blabbermouth friend from high school. You couldn’t tell that friend ANY secrets because they’d just tell everybody they know.