To link your Hub account to Facebook, simply click on "Settings" in the drop-down menu that appears when you click your name in the top right. Then click "Connect Facebook" in the External Authentication Providers section. Once you authorize Health Systems Hub to connect to your Facebook account, you can use Facebook to log in to the Hub in the future.
If you are new to the hub, Facebook can be used to create your new profile. Simply click "Join the Hub" on the Hub homepage, and then click "Connect Using Facebook."