Oddly, Facebook says that you're not allowed to post "Content targeting a person or group of people on the basis of their protected characteristic(s) (in written or visual form): Generalizations that state inferiority in the following ways: Mental characteristics are defined as those about:
Intellectual capacity, including but not limited to: dumb, stupid, idiots.
Mental health, including but not limited to: mentally ill, retarded, crazy, insane."
(It is interesting that "retarded" is classified as being about mental health rather than intellectual capacity)
Notably, many Facebook groups ban use of this word, claiming that it will get them zucced (when in reality it seems only admin violations trigger that). In any event, this comment above was not removed, so.