Via David Stephenson, I found this article about Toronto public health officials using Facebook to find a woman who had been possibly infected with rabies. The ironic part is that the City of Toronto bans the use of Facebook, so she had to get special permission to use it.
When are enterprises going to learn that these sorts of bans are counter productive? People use tools for lots of different reasons. Trying to control people using IT leads to bad feelings, and as this article infers, maybe even death. You get people to do the right thing with good leadership and proper management, not firewalls.