I noticed this as well. I was checking the Facebook developer’s guide on default permissions and they basically state that your site must support the condition that users will now allow their email address to be shared. It’s also the case that users can have a facebook account that doesn’t have an email address associated with it (because they signed up with a phone number).
So on the Discourse side, this needs to be handled more gracefully either by allowing a manually entered address or simply telling the user that they must go back and re-authorize the email address permission.
@Grimbly In order to go back and reset the permissions you gave to the site, you need to login to Facebook and go to Settings - Apps. From there, delete the entry for your Discourse related app.