When you click to logout of Facebook, it doesn't log you out completely. If you come back to the site and login with the quick connect button, your settings are already saved and you will not be prompted for a password.
Using the same pattern in Ryan Bates' Railscast, I've added this modification. Using this, you will be prompted for a password whenever you log back in to Facebook. Note that (at the time of this writing) this will also log you out of your other Facebook accounts.