Back in 1983, 50 corporations controlled the vast majority of all the news media in the U.S. --- and in 1992, fewer than two dozen owned and operated 90% of the mass media. But by 2000, only six corporations owned all the major media.
Recently Jon Stewart complained about the "mainstream" news media. But he's been complaining for years. In 2004 Jon Stewart was on CNN's Crossfire bashing the news media, bashing Crossfire and bashing CNN:
"Stop hurting America. We need your help. Right now you're helping the politicians and corporations. You're not too rough of them, you're part of their strategies. You're partisan hacks. You're doing theater when you should be doing debates. It's not honest. What you do is partisan hackering. You have a responsibility to the public discourse, and you failed miserably."
The media has been failing much worse ever since.
In 2004, in the book The New Media Monopoly, it showed that only 5 huge corporations -- Time Warner, Disney, Murdoch's News Corporation, Bertelsmann of Germany, and Viacom (formerly CBS) controlled most of the media industry in the U.S. --- with General Electric's NBC as a close sixth.
The more concentrated the media becomes, the more they can control the message. Instead of being the government's watchdog, for the last several years the mainstream news outlets have been more like referees between the two two major political parties --- that is, when the media isn't partying with the government --- like at the annual White House Correspondents Dinner (lots of pics).
Some people, like the multi-billionaire Koch brothers, could buy out ALL the media. Scary? Of the world's 100 largest economies, more than half are now global corporations, rather than countries.
UPDATE - From Business
Insider: "These 6 Corporations Control 90% Of The Media In America"
This infographic is from 2012 and is missing some key transactions. Business Insider reported that GE does not own NBC (or Comcast or any media) anymore. So that 6th company is now Comcast. And Time Warner doesn't own AOL, so Huffington Post isn't affiliated with them.