When you get an outstanding kill, or completely embarrass you're opponent. By using the phrase, you are saying that you are going to go to theater mode after the match ends to upload the the clip to Facebook.
It's always best to taunt your opponent, and threaten them that all of Facebook will be able to see them get owned.
Applies to Call of Duty: Modern Warfare 3 players. Due to the new feature that allows players to upload clips from MW3 to Facebook, the phrase was created by AP.
t Knickerbocker: "Wow kid, you just got facebook'd"
NoobWhoDied: "lol wat is guns"
Triple Kill! +250
Whole Team: "Wow those noobs just got Facebooked!"