Drupal services as backend for mobile application

Hi,
I am developing ionic mobile application and i am using drupal as my backend for authentication & content creation.

It is all working great, however; I am now integrating facebook & twitter login to my app which will login the user using his facebook or twitter account.

My question is, how do i authenticate the same user to my drupal back-end using services so i can deal with this user as normal registered drupal user.

I was going to call the service register then login APIs after successful facebook login then i realized i need to double check if the same user register before

Is there some module or documentation that can help me in figuring this out