I agree with most of your post, but on this particular point...
"First they came for the imageboards, and I did not speak out because I did not post on imageboards. Then they came for my facebooks, and zomg how did this happen?"
8chan is just an anonymous imageboard, similar to thousands of others online currently. It happens to have the characteristic of allowing creation of new imageboards in a similar way to Reddit allowing anyone to create subreddits. This characteristic has made it one of the most popular imageboards.
Much like the solution for Reddit was to shut down individual subreddits and ban particular posters, and the solution for Facebook was to close down individual groups and ban particular accounts, the solution for 8chan is to shut down individual boards and ban particular posters.
>Popper's tolerance criteria, by contrast, seems clearcut in a you know it when you see it kind of way.
This breaks down quickly when faced with a current example: Some folks oppose immigration of people that follow religions which espouse intolerance. Via your measure, this is justified. But many I think disagree with that stance, so how can this be resolved?