How to identify a Google OAuth2 user?

Question

I used Facebook login to identify users. When a new user comes, I store their userID in my database. Next time they come, I recognized their Facebook ID and I know which use

Answer 1

"Who is this?" is essentially a service; you have to request access to it as a scope and then make a request to the Google profile resource server to get the identity. See OAuth 2.0 for Login for the details.