Facebook shorthand for "I like this post, but I CBF to type an intelligent comment, so I'll just leave this letter here" ("F" supposedly standing for "follow(ing)", but you can make up your own mind).
FB POSTER: Miley Cyrus total #milf
COMMENTER 1: F
COMMENTER 2: Total failure?
COMMENTER 1: No, I'm following this post, fuckwit!
Commonly used by gamers to respect the fallen or the dead.
Yoda: "Obama's last name is....."
Yoda: *Dies*
Gamers: "F"
F frequently used by gamers is a way to pay respect for your fallen comrad, it is also used to make fun of people for diying in a certain game.
Game: xX._.GamerBoi68419._.Xx was slain by NotABot807
Team-mate: DANG, F in the chat...
Other team-mate HAHAHAH, SUCH A NOOB, F in the chat.
A letter used to pay respects when something tragic happens
Friend #1: Nooo, I lose the game
Friend #2: F