Basically, it means Fuck the Norm. It is usually used in online (or, in some cases, real-life) conversation in response to a query about unusual behaviour.
It can also be a War-Cry like shout of encouragement or sheer rebellion.
"Yeah, well, FTN."
"COME ON! LETS GO! FTN!!!!"
person 1: "Did you hear they finally fired Larry?"
person 2: "What!? FTN!"