I think the answer lay almost entirely with echo chambers. These exist not only in the digital world, but even increasingly in the real world as people connect to people through digital filtering.
Vegetarianism is a good example. Many, though not all, vegetarians are going to be going in that direction due to some form of objection to how animals are treated. But when you surround them with nothing but other people who hold views that are at least as far away from the mean, they're going to suddenly seem quite trite. And this pushes people ever further down one path or another.
By contrast when you have a diverse array of views, the real position of stances can be clearly seen. That is to say for one that agrees that eating meat implies inherent mistreatment of animals, vegetarians are already taking a more active stance than ~97% of Americans. The same would be approximately true for those that e.g. go for fish + veggie diets. But in a world where those 3% are surrounded by only those 3%, vegetarians are extremely 'conservative' in their views.
The whole thing is people need to surround themselves with a good number of people they disagree with on things. But this is not an easy thing to do, and so most people do not. And in fact even things like voting systems on these social media sites discourage such behavior. Many people do not downvote as a matter of something being inappropriate, they downvote because of disagreement. And that pushes views that go against the plurality's grain further and further out of site to the point that people no longer even think they exist. And of course on another site the exact view bias is instead being pushed to the top with everything else being removed out of the groupthink. It's not a very dangerous problem for a democratic nation.