If someone doesn’t have a presence on your preferred social media network, they must either open an account or choose not to access it. You must also support managing your own users, which we’re trying to get away from.

If you choose Facebook, Twitter, Google, GitHub, or any other social media site, you are confined to their scopes. You don’t get to define your own. If the goal is to simply access and leverage user information, this is probably fine. However, if you wish to have managers, board officers, admins, DBAs, and other assorted roles, this won’t suffice.

If your application doesn’t take full advantage of GitHub (for example, https://gitter.im),

then using GitHub probably isn’t the way to go.