Fox News has actually seen a rise in viewership since Obama has taken office.
And expanding on Pariah's comments, O'Reilly has pointed out repeatedly that their viewership includes a considerable number of moderates and liberals who view to see a broader perspective than the liberal-media-Newspeak monopoly that supresses anything that reflects negatively on Obama, and only reports it with the greatest reluctance.
(examples include the BP oil spill, the ACORN scandal, the suppression till now of the Eric Holder/"Fast and Furious" scandal, the Solyndra scandal, the race-focused refusal to prosecute the Black Panthers voter intimidation case, and on and on. In every case, every network but Fox and a few other conservative media have virtually ignored these major scandals. As sharply contrasted with how they leaped on similar stories involving George W. Bush.)
So... is FOX really "partisan"? I would argue they are the only network willing to truly report the facts. They are equal time.