Multiple Facebook Logins

You don’t just have multiple passwords on Facebook, you can log into Facebook using multiple usernames as well.
The default username is the email address that is associated to your Facebook account but you can also login using your Facebook username (that’s in the vanity URL of your Facebook profile page). Similarly, if you have associated your mobile number with Facebook (which you should for security reasons), you can use that mobile number as your username on the Facebook login screen.
A Stack Exchange thread pointed me to this ZDNet story that explains why Facebook allows users to login through multiple passwords. According to Facebook:
We accept three forms of the user’s password to help overcome the most common reasons that authentic logins are rejected. In addition to the original password, we also accept the password if a user inadvertently has caps lock enabled or their mobile device automatically capitalizes the first character of the password.
Thus, if you have accidentally enabled CAPS Lock on the keyboard, the toggled password would still work on Facebook.