True it is the listener in most cases, but when someone like Cena is using/saying/whatever that kind of term its kinda implied. lets look at his other moves.
FU done to mostly guys, occasional female,FU = **** you
STFU done to all guys, STFU = shut the **** up, usually applied from behind while laying on or extremely close to the others anus (yea i said anus it seemed fun to say at the time) as in telling them not to scream from the buttsecks
and as said before five knuckle shuffle, usually with his hand aim at other guys faces.