>ISPs themselves, (older) reddit, 4chan all serve as examples that the people you don't want to talk to can be mostly siloed off to some corner and you can have your own corner where you can have fun. Things only get problematic once you add amplification mechanisms like twitter and facebook feeds or reddit's frontpage.

This isn't true at all, and the reddit report following their ban wave is pretty clear about it; once areas that actively established a standard of violent or racist discourse as acceptable were banned, the volume of objectionable material across the site dropped.

4ch had a similar situation, where the culture on /b/, which was intentionally left as an explicitly unmoderated segment of the site, a silo, actively invaded other boards with violent, racist content.

It isn't that people sit in silos and do nothing otherwise - it's that the silos themselves cause people to believe their content is acceptable, then spread that shit everywhere.