Stands for "If That Makes You Feel Better". Used against people who are dumb and need to constantly feel better about themselves so they say stupid shit.
Aditya: "Hey Krish, I'm so cool I'm better than everyone, you're a loser"
Krish: "itmyfb :)"
Boys who enjoy yaoi (a genre in Japan that contains sexual and/or romantic relations between two men); literally translates to "rotten boy"; corresponding female : fujoshi