Ironic that you've made this comment on HackerNews, a site, it can be argued, whose appeal comes from its pretty substantial moderation. I can't tell you how many links I've seen flagged and banished from HN that I thought were important, but... once I cooled down my outrage, I realized the conversations occurring on them were pretty toxic. I come to HN because of the focus and constructive dialogue. I stay away from Reddit (and long ago abandoned /.) because of the toxicity.

Yes. The alt-right and the_donald reddit community members can move to a less-popular platform, but that doesn't necessarily mean that platform has just become more popular. It means the loudest, most-obnoxious people have just lost their Reddit audience. When 1% of commenters are making 90% of the posts, that's no longer a dialogue and that drives away readers. Encouraging polite dialogue and policing abusive behaviors are smart business moves for Reddit and Twitter.