There was another recent posting - thread now closed - by a nurse who got in trouble over a Facebook posting. I have some direct experience with this - as a union steward - and a fair amount of second hand knowledge. Here are a few basic things to keep in mind:
1. Your Facebook "friends" may very well not be your actual friends - in every case that I've dealt with, management became aware of the offending posting when a "friend" of the poster brought it to their attention. And your privacy settings are no protection.
2. You do not have rights of "free speech" related to your work - the First Amendment protects you against the government intruding on your free speech rights - it says nothing about your employer.
3. If you are not covered by a union contract, in most parts of the US you are an at will employee and can be fired for any reason or no reason - except for a very small number of protected reasons like your age or race.
4. If you post something that would make anyone reading it think they might not get good care at your hospital, that would almost always be a legitimate reason for action against you. Here are a few real life examples: "We were so understaffed it was scary" or "If another person comes into the ER just to get narcs, I might punch them out" or "I got so frustrated with one patient I wanted to take her call light away".
Yeah, we've all had feelings like that, and we might say them to a real friend face to face - but posting them online for all to see - and, no matter what your privacy settings are, it really is for all to see - is just foolish. Save those rants for your truly private conversations.
One last note: Labor law does protect concerted activity. So if your union has a Facebook page and you were having a discussion there about how to improve staffing at your hospital, a negative comment on staffing in that context would likely be protected by law - but the exact same comment in the context of just a general rant likely would not be.
So, be careful, be smart, don't cause yourself unneeded trouble for silly reasons.