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.