First you would get the facebook login working by enabling Janrain Engage (formerly RPX) and making Facebook the only enabled provider in your Janrain account. It requires a facebook app id entered in your Janrain account to get it working.

Then after registering your own user using your facebook credentials, add that user to the admins role so you can manage the site. Then you can disable database authentication from Site Settings. Once you do that the only way to login will be facebook. If you get yourself locked out by disabling db authentication the only way to fix it is by direct access to the db and the mp_Sites table where you could re-enable db auth.

Hope that helps,

Joe