American democracy of fifty years ago had the mainstream press as gatekeepers. At the same time, you had a measure of the press taking it's job seriously, the state being separate from private industry and the education system producing an informed populace and the middle class having an material stake in the running of the society.
Today you see a decline in all this stuff alone with the rise of the Internet. Phenomena like anti-Vax movements and other conspiracy theories are certainly corrosive but if one is looking for analogies to this, one might look at the problematic politics of third world nations where wild rumors and demagogues have been a characterizing factor for a long time. And all the mainstream institutions of the US certain bear blame for allowing the "third-world-ization" of the US.