8chan's /pol/ is much larger than /leftypol/. I believe it's about tied with /v/, and those two are the largest boards. There are many basically innocent parts of 8chan, but /pol/ makes up a significant fraction of the site's activity, maybe as much as a third.

/pol/ is full of people who encourage shootings, and the website's and board's rules allow people to encourage shootings. You're not allowed to encourage shootings anywhere on Facebook, I think. That's a very significant difference between the two.

Do you have much experience with the chans? I've spent an embarrassing amount of time on them over the years, and my experience is that it would be ridiculous to compare the ratios of politically hateful, extremist content to normal content between Facebook and the chans. The difference is night and day. The chans are full of great, diverse content, but an absolutely solid core of their identity is inevitably dominated by the worst elements of society at any given time - it's just inevitable due to the nature of the format and rules.

The media will tend to focus on the part of any site that promotes and radicalizes participants into mass murder.

Facebook, as you know, moved extremely aggressively to remove the NZ shooting video, along with other extremist content. Basically Facebook employs a raft of people to prevent the thing that 8chan was trying to encourage.

Again, 8chan is not one person, most users dont encourage this at all because it brings problems like exactly this one. Its the price of a truly free platform, sadly it seems americans hate freedom.

In the practical sense, I doubt this will counter shootings at all, intelligence services had them contained in a single board, now they will just spill around the internet.