>That's because this stuff was a tiny fraction of the activity on Facebook (it's HUGE, and still mostly for baby pictures), but a substantial fraction of the content on Parler and Gab (they're tiny, and mostly for the stuff that Facebook and Twitter ban).
There's no source, but it stands to reason: Facebook has something like 2 billion daily active users and it its roots are in nonpolitical social networking (meaning baby pictures, etc.). Parler had something like 2-3 million daily active users at its peak [1], and most of them joined after the Twitter started putting warning labels on false claims of election fraud. Parler also billed itself as a "free speech" social network, which in practice means allowing things other social networks prohibit, which means its mainly gets users who want to post and read such stuff [2].
I would love to see a source for this claim.