Yes, if it falls into the category of lashon horah, (derogatory or causing loss) then it’s lashon hora even (maybe even more) when posting. The fact that it’s in public domain makes it a lot worse.
However, in my VERY humble opinion, it should not be applied to posters – one on another – since we are anonymous. (most of us..) In our case, I think, the laws of Ona’as Devarim apply. Even in anonymity one can hurt another’s feelings. I don’t think it happens here, just pointing it out!
– Correct me if I’m wrong –