Users can sign in to your B4A or B4i apps using Google or Facebook with the FirebaseAuth libraries. FirebaseServer completes the puzzle with server side verification of the user. This means that the signed in user gets a token id (long string) from Firebase services by calling...
Sub fs_TokenVerified (TokenId As String, Success As Boolean, Token As FirebaseToken)
If Success Then
Log(Token.DisplayName)
Log(Token.Email)
Log(Token.Uid)
End If
End Sub
It is up to you to store the successfully check of a token in your DB to do it only if not already verified.