What are some criticisms of our so-called 'fourth branch of government?' How do the media effect public opinion?

Different media companies affect public opinion differently. In surveys year after year, Fox News viewers are consistently the most misinformed, while NPR listeners tend to be the best informed. that's a pretty significant effect.

STUDY: Watching Only Fox News Makes You Less Informed Than Watching No News At All http://www.businessinsider.com/study-watching-fox-news-makes-you-less-informed-than-watching-no-news-at-all-2012-5

Seven Surveys Make A Trend For Fox And Viewers http://mediamatters.org/blog/2011/11/22/seven-surveys-make-a-trend-for-fox-and-viewers/167217

Fox News Viewers Uninformed, NPR Listeners Not, Poll Suggests http://www.forbes.com/sites/kenrapoza/2011/11/21/fox-news-viewers-uninformed-npr-listeners-not-poll-suggests/