[CyanLite2]

Simple: "Ignorant" people make up a large percentage of their user base. They argue with "informed" people and it drives traffic and revenue.

Meanwhile the social fabric of the United States is being torn apart by political polarization.

[Nextgrid]

Ignorant people are also more likely to click on whatever ads are shown without any skepticism.

[malandrew]

This polarization certainly predates facebook:

https://www.youtube.com/watch?v=tEczkhfLwqM