Which automatic post are you asking about? If you have the "Push Social Network Status:" setting enabled in the JFBConnect -> Profiles area, when the user logs in using Facebook, they will get an additional prompt asking for permission to post on their behalf. That's a permission that Facebook asks and will be in an additional popup.
Once that's approved, any public posts made into JomSocial should be posted to Facebook. If the user changes the settings to anything other than 'Public', we will not post it to Facebook.
I hope that explains, but if not, please let us know exactly what you're looking to do.
Thanks,
Alex