Sigh... how hard did you try? Haven't you ever heard of
astroturfing?
Agent of influence - Wikipedia, the free encyclopedia
An example of a government having a fleet of propagandists that post online:
Russia's Online-Comment Propaganda Army - The Atlantic
There are only a few
known cases of the U.S. government doing it directly, like
this one. Mostly, it relies on the corporate sector to tow the line, since the two entities are so intertwined at this point. Government is business and business is government.
Here's a nice info graphic:
These 6 Corporations Control 90% Of The Media In America - Business Insider
The media is controlled by a handful of corporations, some of whom are staffed by former members of government or vice versa. The FDA is also full of cronyism. The government may not issue edicts censoring or banning media directly, but it just takes a quick phone call to a corporate "partner" or business associate to get a story changed. It happens all the time and is referred to as just doing business or "public relations". If someone important enough does not want a portion of a story to go to air, it gets cut. These kinds of political favors happen all the time. It's not conspiracy, it's reality.
As for media polarization, here is the largest study ever done on the issue:
Political Polarization in the American Public | Pew Research Center for the People and the Press
America is more polarized than ever before. Is it any wonder? All the media does is create a false dilemma between two opposing views and then pits them against one another. Real information and critical thinking in the media requires abstract thought and complex reasoning, something that our media has abandoned in favor of sensationalism. That, and most of our media corporations are part of larger conglomerations that have ties to the military-industrial complex, so it's in their interest to keep the war machine going. Keep the sheep riled up and they won't bother checking into the real story.
Now... I've done enough work for you. I pulled up these links in about 10 minutes of searching. Try harder next time.
You're welcome!