User login API for social login from mobile apps
I was also looking for functionality that you needs, but after wards i tried to implement service for social sign in using Hybridauth module and i succeed.

Try this https://github.com/rathodhiren9/hybridauth-service.git

Note : Its my own implementation and working fine using Poster/Postman. But you need to test in mobile application on your own.  


Place this module folder in /sites/all/modules/custom and give full permissions if you are using Linux/Unix.
Look for Hybridauth Services in you all module list from Admin pannel of drupal 7 and install/activate it.
Now goto structure/services and add new endpoint if you don't have (remember Path to endpoint will be used in your URL call of service from mobile app).





After that you need to add Resources as per you requirement and for this service you need to check "hybridauth" resource and "connect" action from available resources and save the configuration.
Now this service will work from URL like /domainname/path-to-endpoint/hybridauth/connect. Be sure that url of your domain must match with your social media app's url that you are using for facebook or google
If your done well all the steps then you surly will test the service from Postman or Poster tools, But you need to get access token which will be an array of user data came from Facebook or Google and you will easily get it by debugging Hybridauth module's hybridauth.pages.inc file.


Hope this may be helpful.