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This is short for face to face. It is used frequently in email, on irc, etc. It refers to having a conversation with someone, in the same room - talking.

We frequently used term at work in email, to let the person know that if we need to talk to them, that the telephone isn't good enough.

Example: "Get a hold of me later - f2f", or "He is so concise in email, but f2f he rambles".