I have not personally worked on the Google /Facebook account to authenticate user . I think you will have to find out the API which authenticates the user and sens the User ID as header information.(you can try Securely call Google APIs with OAuth 2.0 authorization.).
After authentication once you receive the header information you can parse the header information in Pega Authentication activity(Called from Authentication Service) for Authorization/login/Id creation in Pega on the fly.
From Pega end you may creat a Servlet mapping dedicated to this authentication and call the Authentication servivce name from there which internally calls the Authentication activity.
To get more information on Google authentication API: visit this page