Of course you know you can link people to /login or /account (or for that matter anywhere in /account/...) where you can require someone to login via username/password, Facebook or Twitter (or register a new account) in order to continue.  In the specific case you want to route them directly to Facebook or Twitter, then follow these techniques:

Route to Facebook for Authentication

You can route a user to Facebook to authenticate.  Once they successfully authenticate, we will route them back to the page they came from.

Link to: /login/facebook

Route to Twitter for Authentication

You can route a user to Twitter to authenticate.  Once they successfully authenticate, we will route them back to the page they came from.

Link to: /login/twitter

Additional Paths Supported