I can log in via Facebook in Firefox, but not Chrome, Edge, or Safari. For what it’s worth, I’m using Firefox’s Facebook Container extension to keep Facebook from tracking me around the internet - whether or not that makes a difference, I’m not sure. We’re looking into it.
Probably related to the fix for this: https://forum.inaturalist.org/t/sign-in-with-google-doesnt-work/17644/13
You can create a password for your account via https://www.inaturalist.org/users/password/new then log in using your username/email address and password. Then you don’t have to rely on third party log in.