There was a massive article in one of the red tops a few months back, "FACEBOOK GROUP HORROR!", it showed openly public groups of shop assistants talking about how they hated certain people and horrible things they had done to products.
Safe to say some people got fired. The nameless company I work for sent out a massive letter about it, basically saying anything that can be directly link to you, that effects the company blah blah...gross misconduct, fired!
O man it's well scary, there was that 1 of that women who went on Facebook during work time saying she was bored and the boss noticed the time stamp that she was at work and sacked her, always turn chat off at home if your going to use it at work as well, so many ppl at my work get caught with that by management.