With the release of the apostrophe-passport module, it has become very easy to implement login via Google, Twitter, Facebook, Github, Gitlab, just about any site that supports OpenAuth, and many other solutions such as SAML. This capability is often known as federation or "single sign-on."
See the apostrophe-passport documentation for complete examples of Gitlab and Google login configuration.
The same techniques can be applied to Twitter, Facebook, and almost any other authentication provider that has a Passport strategy module. The `apostrophe-passport' module is a "universal bridge" compatible with pretty much all Passport strategies that follow the best practices laid down by Jared Hanson, the author of Passport.