flame - on messageboards, the posting of comment or replies ranging from the purely childish to the splendidly witty, and hitting every mark in between. Usage example: "Warning: flaming is allowed on this board".
F2F - face to face; in RL; in the real world. Usage example: "I'm meeting my online gf f2f next month. I'm really nervous. What if I don't like her in real life? What if she doesn't like me?".
FAQ - frequently asked questions.
FOAD - fuck off and die.
FYI - for your information.