I have it set to full joomla user and not skip activation.
How I see it working is a first time user hits the login with facebook button and when they have the option of connecting their account or registering for a new account which in my case will be a jomsocial user. There would be a password field instead of the JFBC registration field. Once they input the password, like a multi-part form opens up to show the registration form as it normally would. If they input the wrong code then it gives a warning and does not let them register.
Or simply just have the first field with text box that must have the password to send the registration form. Otherwise if they are not registering an account the facebook button login would work like normal.
The reason for this is I have a family site that I want to make as easy as possible for family to use and most are facebook users. But it will contain private info that I don't want the public to see. Using something like a password to prevent unauthorized poeple from registering seems to be easiest without have to manually create each user or approve each member.
I have attached a screen shot of a basic idea.