Check out Social Login package to enable multiple connections (Facebook, Twitter, LinkedIn, FourSquare, Google+, ecc.).
This package adds a block containing following features:
- a "Connect with facebook" link (you can customize it in the view) that lets the user directly connect to the site without registration
- if the facebook email is already in db or the user is already signed in the facebook account is "connected" with the user account
- if it's not, the user is automatically registered as new user and an email is sent to the user with username and password.
You can follow these guidelines to create your application and set the api keys.