It seems a spectacularly bad idea under most circumstances. Even in situations where both parties consented and it was obviously just a bad taste joke between friends, it would still likely be a pretty poor plan because of the very public nature of the medium.

Edit: And, hmmm, I guess that most people would probably find it socially acceptable to e.g. wish ill on e.g. a murderer, a terrorist, a dictator etc. It isn't something I would do personally, but I have observed that other people do so (even if they would otherwise be horrified at wishing ill on people)