This is interesting. You can log into your Facebook account using three passwords – one is the main password that you created and the other passwords can be constructed by toggling the case of characters that form your original password.

For example, if your Facebook password is operati@nGeronimo!, you can convert the uppercase characters to lowercase (and vice-versa) and the toggled password string – OPERATI@NgERONIMO! – will also get you into Facebook.

There’s another variation as well. If the first character of your Facebook password is in small case, just change that character to upper case – like Operati@nGeronimo! - and Facebook will gladly let you in.
You may try these variations with your own password on the Facebook login screen available at facebook.com/login.php.

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.
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.