Hey Clive,
Here on compojoom we also use socialconnect. But when you click to create a new account you are being redirected to the standard joomla user manager.
This is how our login page looks:
compojoom.com/login
When you click on register now you are redirected to com_users and you'll see that we have a newsletter option there.
If you want to somehow force the user to select if they want to subscribe to the newsletter and then register through facebook - this is not possible. What you could do is - show them the newsletter module after the login. If they have an account the module could be already prefilled with their email and name so what is left is just to click subscribe.
Regards,
Daniel