When facts are not on your side, you're reduced to the shit you offer.
(A little dated but best I could find.)
So FNC is in as many homes as pretty much any cable news network and enjoys gangbuster ratings. Yet plays the victim card of "We're outside the mainstream" because of the idiocy that is "Conservatism cannot fail, it can only be failed."
By the definition you offered, FNC is a mainstream media source. Saying otherwise is disingenuous.