This article has a great list of things people posted to Facebook that got them fired. To me, the most outrageous is the one where the prison guard posted about an assault on a prisoner and other guards chimed in with their support. This got them all fired.
The articles goes on to discuss the need for companies to provide social networking policies but I will harp on my pet peeve which is that employees should receive basic training on the use and misuse of social networking In the case of the guards, they may think that their Facebook conversation is as ephemeral and private as chatting around the water cooler. A good training exercise would quickly show people how this isn’t the case.