Auth0 provides authentication and authorization as a service.
Demo
You can see the demo of the functionality on this page (protected with "username"/"password"). The read-only editor can be found here. Look under Plugins > API Connector, and expand the API that is named "Auth0".
Implementation
This implementation leverages Bubble's API connector's OAuth2 User-Agent Flow.
1. Sign up for an Auth0 account and create an app
2. Set your logo, application type, authentication method and callback URL
3. Copy your auth0 domain, client ID and client secret
4. Set up your API in the API connector
5. Add a "signup/login with a social network" workflow
