By default a new facebook registered user will be placed in the role "subscriber" so it's possible that that role doesn't exist due to another plugin e.g. custom role plugin if not I would be interested to know why it's doing this. In your WPBB facebook settings you'll see a setting like "Default Facebook Role". Enter the role you want all facebook registered users to fall into (it's case insensitive). You can also email me at firstname.lastname@example.org if this doesn't work and need to send me a link to your website / temporary admin account etc. Thanks.