My policy, that has served me well, is that I never, under any circumstance, add coworkers to my Facebook "Friends List" until I have left the job.
I can't see any good coming out of adding anyone while we are still coworkers. I don't need anyone monitoring my every move and I don't like having to think too hard about who is in what group when I am sharing opinions and personal thoughts.
What if I decide to post that I'm heading to an interview (which I do occasionally just to test the waters) or if another friend asks me about a recent interview experienc?
Can everyone I work with be trusted with that info? Even if one person can be trusted, they are like a vector by which another (untrustworthy/gossipping) employee could gain access to information I provide.
What happens when a nosy or hostile coworker steals a glance at their phone FB wall while they are distracted etc.
No coworkers...ever...this works for me since I tend to change jobs every 2-3 years anyhow. While I'm at a job, if we really are friends, then we probably don't communicate primarily through FB. If we are friends, we will have each others phone numbers and can text/call/im/email to arrange pretty much anything.