Well, in that scenario Joe's co-worker, Average Bob, tweets that Joe sucks at his job and will be canned. The notion a co-worker thinks someone sucks at his job is pedestrian. Tweeting it far and wide would be juvenile, extreme, and very likely just stupid, but not impossible and the anonymity thing might embolden. Still, if Average Bob really values his job and wants to have a professional reputation, trashing Joe publicly would be just as likely to backfire in Bob's face as to get Joe sunk.
So, yeah, it's pretty loopy as well.