The portal users can log in with any of three social media platforms if you have enabled the Customer Onboarding Setting from the Settings → Portal Settings and configured their keys individually. The portal users will get the social media buttons that are enabled.
By clicking on any social media button the portal users will be authenticated by the social media login if the portal user is not registered, they will be required to register first. So, the portal users can log in with user validation if you have enabled it from the Customer Onboarding User Validation settings.
Note: If any of the social logins do not have the Email field then the additional Email field section appears and the portal user needs to enter the Email.