I am certainly not a lawyer, though I have come to find the law and the logic of thought and arguement very interesting...
Anyway I remember a certain actual lawyer once writing to someone that "The truth is the best defense to an action in slander" when that accusation was made. If a claim is true, by definition, I don't belive it can either be slander (spoken) or libel (print).
When it comes to work related stuff, I think it makes a huge difference whether you work under a union contract or are simply empolyed "at will." I remember reading in the paper once (business section - 'on the job' advice column) that an "at will" employee can be fired for nothing more than the powers that be just don't like the person - no real explanation needs to be given, so long as the firing is not due to the employee being part of some legally protected class such as race, nationality, etc. (Some would argue, in defense of this freedom, that the employee is likewise free to quit his job wiout having to justify himself.) Now if you are under a union contract, as the case with labor at UPS, you have far more protection.
So as I understand things, if one were to post on facebook "I hate working at UPS. I despise all the overtime which I do not want, and my on car sup, Tom Jones (his hypothetical real name) I find to be a complete jerk," I don't think one would have committed a legitimately disciplinable offense. I would advise anyone facing discipline at UPS for such a comment to examine closely what his rights are. Now if a comparable statemnt were made by a Walmart employee, for example, where there is no union and you are employed "at will," I belive you could be fired and would have no recourse.
The legal limitations one needs to be mindful of as a UPS employee would be things like conspiring on facebook to do something illegal - like go on a shooting rampage at in the hub, or sharing something that is in some way proprietary. That could include something like having your fb picture be of you in uniform by the ups truck you drive (company property), or better yet something like a picture of all your packages having fallen off the shelves into a disasterous pile in the back.