The mainstream media in the U.S. have had increasing credibility problems in recent years. Recent events have suddenly made clear to a much wider audience just how slavishly they are tied to both the "intelligence community" and "the Establishment." Their inability to grasp how politically powerful was Donald Trump's unprecedented invoking of reality left them off balance. This was a serious problem for an industry as concentrated as the mainstream media, with no diversity to restore that balance. Their response thus was to simply deny the reality, and the tool with which they chose to do this was to label its manifestations as "Fake News."
It is a direct analog of the use of the term "Conspiracy Theory" to shut down any conversation that might provide an insightful glimpse into the undesired reality.
The initial purveyors of this idea were those traditionally associated with truly fake news -- the New York Times, the Washington Post, Fox News, CNN, and PBS, loaded down with such baggage as
The term has now spread to the primarily foundation-funded mainstream "alternative" media and -- more dangerously -- the blacklisting and suppression of "fake" news sites by Google, Facebook, and Twitter. That is to say, anyone or any entity that finds a particular piece of news inconvenient -- such as the Democratic Party trying to explain their loss (actually, a theft) of an election for which their campaign was crippled from the start by being joined at the hip with a badly flawed candidate.