> they get radicalized by repeated exposure to extreme viewpoints
I don't think this holds up to any scrutiny to be honest. There are certainly some cases where people are easily influenced, but I don't think it is that significant. Only from a worldview that puts you way above others. Aside from the problem of defining extremism, people do make choices.
I think what many people observe about conduct on social media is a form of desensitization for that matter and there are different reasons for that.
But this is actually testable, by identifying this propaganda and identifying people affected by it. We had that topic with Russians that supposedly influenced a massive amount of people with extremism for an election. But if propaganda is in place, you can identify it by words, images or statements. You can see this with ISIS. There were some poeple that indeed fell for it, but probably not by facebook adds. These people were actively groomed.
Wanting to control content by large ad corporations can also be seen as extremism, especially in the context of the free internet.