Jérôme Mouneyrac has updated perhaps the most robust authentication plugin for Moodle which supports 7 different authentication sources including Facebook, Google, and LinkedIn adding a very simple and easy registration option for new users on your site (if the site is open to self-registration).
The posted installation process looks both simple and easy to follow (just three steps). Once installed choose which Auth options you want to employ on your site.
Download it here: https://moodle.org/plugins/view/auth_googleoauth2
Thank you Joseph for this very nice post. It makes me want to work more on it when I read your post. My next tasks: add twitter and facebook anonymous authentication, redesign the display, automatically download pictures, a more OOP code and maybe other arguable features like allowing users to connect on existing email address 🙂 Cheers.