I think its the echo chamber nature of social media. Because its not just flat earth, its every crackpot conspiracy theory. Flat earth, anti-vaxers, moon deniers, chemtrails. All generally existed prior to social media... but with facebook and twitter letting you create an echo chamber of "liked" and "followed" content more people are "out of the closet" with their crazy b.s. Because inside the echo chamber it seems like more people agree with you than those that actually do. Its why Trump and Bernie Sanders and Hillary Clinton supporters all seem to think that the majority is on "their" side. Its not the case, in actuality the electorate was as or more divided than it ever has been. But inside everyone's echo chamber... it seems like everyone agrees with you.