According the Facebook documentation for "display modes" (https://developers.facebook.com/docs/reference/dialogs/), the "popup" attribute does not automatically create your popup for you, instead it just formats content on the page such that the content looks best in a 'popup'
They key wording is "For use in a browser popup no bigger than 400px by 580px". They are suggesting you use javascript to invoke a popup and format that popup no larger than 400 by 580
facebook_share_url = 'https://www.facebook.com/dialog/feed?app_id='+ window.settings_fb_key +'&link='+ merchant_url +'&redirect_uri='+ merchant_url +'&display=popup'window.open(facebook_share_url,'','width=400,height=580')