Ben Compaine cites a National Science Foundation-funded study showing that it's the marketplace, not the ownership, that drives "slant" in news media. This makes perfect sense in a Darwinian way. Iconoclasts and polemicists may have their audiences, but you can't drift too far from the consensus and still maintain a big audience. So writers write and editors edit to the expectations of the community.
With the Internet and hundreds of globally distributed cable/satellite video channels, it's now possible to aggregate significant numbers of people -- not a majority, but big numbers nonetheless -- by viewpoint and interest, not just by geography as newspapers did a century ago. So we have a redefinition of "community" and some ideologically segmented marketing. Voila: Fox News. It's not just Rupert Murdoch and Roger Ailes who are responsible for it; there's a significant market, or it wouldn't exist.