Paul,
There's no way to have a user subscribe to newsletters without creating an account. JFBConnect authentication is meant to register a user on your site. During registration, the AcyMailing plugins for subscription on registration will work though, and the user can be automatically signed up to a specific list if you'd like.
There's no way to show a "Facebook" button to guests that, when clicked, will simply add their name and email address to your list. It's a nice idea, but not something we support.
I hope that helps explain, but if you were looking for something else, or need any other help, just let me know.
Thanks,
Alex