Exactly what you think it is: a fuck you emoticon. The computer doesn't have a face for it...yet. Used when you hate something or someone and you can't put it in words.
<person 1> yo dood we goin to get fucked up meng
<person 2> yo sick bro, where?
<person 1> nah you're not invited
<person 2> :FUCKYOU
<person 1> ......how the fuck did you do that face?