Oh, I see where you're coming from. You want to define "mainstream" as "number of channels," even though Fox has the majority of the audience. That's a bit silly as an argument for TV influence, don't you think? A thousand people can yell and shout, but it hardly matters if everyone's listening to one other guy.
More broadly, the accusation about the "mainstream media" being against conservatives is just a broad case of conservatives playing the victim to justify their views.