Interest-driven communities online paired with censorship lead people to increasingly radical points of view. Before social media, people were forced to interact and get along with people with varying points of view. Now, we choose to isolate ourselves into groups of like-minded individuals where dissent is silenced, leading people to believe that extremism is normal.
It's a matter of scale; before social media, small groups would be isolated due to difficulty in "finding" each other. Now it's just a quick google search away.
The other part of that "scale" is that these fringe views are presented with the same facade of authority as a more mainstream view. Generally speaking, something like Google will present the search results in a non-biased way... so googling for "flat earth" will bring up links, just as "moon landing" would. So to a naive viewer, "Google" is approving of the idea of a "flat earth", if that makes sense...
