More significantly, how do ensure I don't flash someone's inappropriate party pictures at work?
It's not a sterile environment and you can't control what's there. Either employers will understand that and not care that you're on Facebook during a short break, or they won't understand and will be upset that you're on Facebook in the first place.
*if you see what I did there.
That being said, I would imagine this is an optional thing..
Quite a few people use Facebook at work. Theoretically Facebook wants to keep them as customers.