This isn't a issue really its just a warning. Meteor doesn't use the JS Facebook SDK and gives a specified height/width for the facebook OAuth dialog & your browser size may be too big for the small popup (not too sure- looks ok in your screenshot).
If you disable your app from Sandbox mode/another user uses your app they wouldn't see this warning anymore.
The alternative is to use a redirect to facebook or a mobile display. The best option is the popup (what you have now) because it's the fastest: A redirect means your Meteor app has to reconnect up.