I don't have any documentation yet, but each of the plugins use a different form of authentication and are quite small if you want to go through the code. There is a good chance that your social site already uses one our existing methods so the adaptation should be easy:
Twitter Plugin: This uses OAuth 1. Facebook Plugin: This uses OAuth 2. OpenID Plugin: This uses OpenID. Google Signin Plugin: This also uses OpenID, but makes use of the above OpenID plugin.
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Twitter Plugin: This uses OAuth 1.
Facebook Plugin: This uses OAuth 2.
OpenID Plugin: This uses OpenID.
Google Signin Plugin: This also uses OpenID, but makes use of the above OpenID plugin.