There are actually a bunch of studies that explore the role of mass media, in particular the radio, in the spread of Nazi propaganda in the years before Hitler came to power.
The radio played a big role in the Rwandan genocide as well.
Mass media was central to fascism because fascism requires the ability to build collective consensus over a broad geography that wasn't easy before mass media. It's hard to otherwise align the morality of millions of people.
I share the concern that social media will be like this, but more potent. And it won't just be a distribution channel, it'll be a breeding ground for the ideas that create extremist ideologies in the first place. It's like the wuhan wet market, but for ideas. The ideas that come from this breeding ground are not necessarily good ideas, just popular ones. And fascism is one subset of bad ideas that have been historically popular.
But I'm interested to hear what you have to say about that. Why did social media enable fascism now, and why did fascism exist before it?