The latest story to hit the headlines about Facebook concerns comments made by Essex police not realising they could be seen by members of the public. The officers made inappropiate comments which are now being investigated as misconduct. You can read the full story here.
The rules about Facebook it seems to me are simple. Do not publish any content that you do not want your boss, your colleagues or customers (in this case the public) to see. Facebook although with varying security settings is no different than any other website in this regard.
As people are finding time and time again, not following the rules can land them in hot water with their employer. We have reported incidents of dismissals for comments on facebook and have seen content used in employment tribunal cases.