A Mariah Carey forum filled with Mariah booty lickers, who love anything she does, and they also think Mariah Carey can do no wrong. And if you think so other wise, or don't agree with their opinion, you get banned. Obviously they forgot the meaning of the message Mariah posted there a while back.
FOMM (Friends of Mariah Messageboard) are Mariah Carey bootylickers.
Very uncommonly used abbreviation meaning "Fuck Off My Messenger", not to be onfused with FOML ("Fuck Off My Lawn"), it is definitely of internet-only usage.
Bob: Dude, your pic looks like a granny's butt cheek.
Bill: FOMM you stupid n00b!