This is the emoticon expression of when one holds one's hand in front of one's face, palm towards the face, and pulls an imaginary football face mask downward, while saying "Face" or "Facial" or "Facialization."
In real life, "Face" is used as an emphatic way to communicate "Burn", or "I told you so."
For further explanation of Face, look up "Face."
Joe: I got the highest score on Asteroids.
Tim: I just beat it! x)!
- or -
Before the game:
Bob: The 49'ers are going to win.
Dan: I would go with the Colts.
After the game:
Bob: Did you see the game?
Dan: x) I told you the Colts would win.
A kind of mischevious.. looking.. smiley.. See the sinister kind of plotting smirk!? Can't you see it?
Me: This should be a good RP.. x)
Other: Oh god, what are you plotting?
Me: ..nooothinnnggg... x)