A lot of social media users have been wondering why their feeds have been filled with far-right, often racist, content. For me, it started with more conservative media outlets and Republican politicians in my feed. I’m a journalist who covers this stuff, so at first it wasn’t so alarming.

But then came more and more memes about Islam—the religion in its totality—being incompatible with the West, and the need for its expulsion. Then there were the grotesque caricatures of Black people. And then came the antisemitism in the comments: praise for “the painter” (Adolf Hitler), “109/110” (the hoax that Jews have been kicked out of 109 countries, and it will be soon be 110), and blaming everything from economic downturn to falling birth rates on a juice-box emoji (sounds like “Jews”)...