Matt

Provide one click single sign-on login and automatic registration support for your user’s Facebook account. OneLogin is a decentralized framework that comes with support for Facebook. Any oAuth service provider could be used with minimal effort.
OneLogin provides a system for third-party plugins and theme functions to request additional permissions or use any Facebook feature.
Unlike other social login plugins all processing is done on your site.

OneLogin Single Sign-on

Provide one click single sign-on login and automatic registration support for your user's Facebook account. All processing is done on your site.

OneLogin Single Sign-on vs Registrations for the Events Calendar – Event Registration Plugin