But they DON'T USE React Native in the main feeds of either Instagram or Facebook. You know, where users spend all their time and performance matters.

You linked to a post about a bunch of settings/picker screens in Insta. On the FB side, my understanding is that only the Marketplace tab is RN, and maybe the settings & notifications.

oh so that's why each time I reopen an instagram convo from the background, I have a blank webpage .. thanks RN !