You can have any number of authentications on your account. Adding more authentications makes it easy to log in through whichever social accounts you currently are logged into. Available authentications are through Facebook, Twitter, and Steam.
To add an authentication, go to your authentications page, and select the authentication that you would like to add. Log in through the service, and it will become available for you to use as a login method in the future.
To remove an authentication, click on the × beside the authentication you want to remove.