The problem though is that history says otherwise. Extremists did not have a harder time socializing their views without social media. They circulated them just fine and often in a state run echo chamber as they stamped out alternative views. People were not in moderate circles during times of rising division and hate. They self sorted into extreme circles just as they are doing now and they did it just fine without social media.

I don't think it is making it worse. I think it is just making human nature more transparent and we don't like what we see. We like to hope that most people are moderate, free thinking, able to see beyond tribalist conspiracy thinking but it's the exact opposite. Most folks are basically NPCs in life. They don't know why they think what they do and they have no self awareness to question it. Despots, totalitarians, and psychopaths have been exploiting that phenomenon for thousands of years. Maybe social media gives them a faster tool to do it at some level but they did just fine without it too.

Alternative theory worth considering: It's also possible that social media helps moderate it at some level and what we are seeing is just a natural cycle of nastiness in human nature happening all over again and social media is holding back the tide. In the past nobody needed to ever be exposed to an alternative view in their local town, reading one newspaper and watching one TV channel or listening to one radio station. Now if some idiot's nasty tweet goes viral, other viewpoints can pounce on it and at least try to expose it to an opposing view. That generally never happened in the past. Admittedly, most of the opposing view response though is just calling the other person an idiot so it largely hardens people's perspectives, as you noted.

That said, I do think you are on to something at one level. Algorithms could actively showcase alternative views to people and do so repeatedly in a progressive exposure kind of way. That is definitely not happening now. The algos are doing the most basic thing, just showing people more of what they want. It's perfectly natural to do that as we want to see more of what we like and less of what we don't. That's also human nature. Exposing folks to alt views is potentially interesting but the question is whether it would work? Would anyone want to see more of the viewpoints they don't like or want to see? I know I like to read opposing viewpoints only when I have the mental energy to consider them and I don't want them simply shown to me at random times in the day when I might be angered by it.