So, all of the sudden, after switching from Safari to Chrome and trying twice, the Facebook login suddenly worked. Hopefully it keeps working now. That "state" parameter in the callback URL seems to be very finicky and sensitive. And I think it could also have to do with the PWA extension, or maybe any extension that messes with the login/session. Or, could be anything that messes with caching or cookies (aka PWA extension? maybe?) I have no idea.
But, the Google log in still gives me invalid state.
I also worry that the Facebook login may be suddenly working for me, but who knows if it will work for a random user.
Twitter login still doesn't work either.
I personally won't be able to figure it out. An experience developer with a lot of OAuth experience would need to throughly test this extension in different browsers, with the PWA and a bunch of other extensions. It's not a big deal for me personally... I'm just suggesting this be done for the sake of others. 🙂