Support logging in using services other than Google

[punchagan]

Auth0's free plan could work well for this. It allows for 7000 active users, but only allows for 2 of the social connections, from the providers it supports below.
https://auth0.com/docs/connections/identity-providers-social

[punchagan]

It seemed specially interesting because they provide the passwordless authentication mechanism - though SMS doesn't work (without our own Twilio credentials)!

They also don't seem to make life easier to manage the "Applications" on all these platforms, and client IDs and secrets.

It might just be easier to use Flask-dance and add blueprint that connects to Facebook, and be done with it!