I like this explanation and the one in the parent comment quite a bit. Rather than a grand thesis about internet / social media [0] / polarization that one's likely to see in a think-piece, it's the little things about how we've chosen to structure our society. These things that seem like trivial inconveniences [1], like just making some new friends when yours have moved away, or if you live the suburbs just taking that 20 minute drive whenever you want to see someone, end up having a disproportionate impact on our behaviour. After all, we evolved in an environment where one was pretty much automatically embedded in a community, with very little effort required to socialize other than following the social norms of the tribe.
One other thing I'd also point to is network effects. If lots of people are all finding it easier to be online all the time rather than be social, then for any given person, that results in a reduced number of potential friends.
[0] I would actually put some blame on social media for being a poor substitute for socialization, but just good enough that it makes people less likely to dig themselves out of being isolated. But blaming the internet in general, including things like blogs makes as little sense as blaming books.
[1] https://www.lesswrong.com/posts/reitXJgJXFzKpdKyd/beware-tri...