If you access Facebook at work and post shit about people at work, you're a moron.
If you post shit about people at work and your profile is not private, you're a moron.
I don't even bother checking my email or any site for recreational purposes on work computers. I send stuff for work to my email and download it at work (RTF files), but that's it. Even then I'm horrified someone watching the network will see the titles of other emails in my inbox and I'll get in trouble.
However, laptops with personal aircards are another matter altogether. Just gotta make sure you hear the footsteps coming.