Agreed. Especially if you are posting complaints about your boss, coworkers, etc. While I don't agree it should be a fire-able offense, it's just plain stupid. if you are using your real name for your account.
Now, the teacher posting pics with drinks in her hand is a different story. That is perfectly acceptable and only a moron would think to use it against her. The only negative thing there is that she actually has, and uses, a Facebook account.