I disabled hotlinking via CPanel, and added my own website, plus my CDN, www.facebook.com, facebook.com to the exceptions list for both https and http protocols.
However, when I use the Facebook debugger https://developers.facebook.com/tools/debug, it often (not always) removes the featured image and simply shows a grey box where the image should be. It also displays the message the image
"could not be processed as an image because it has an invalid content type".
The debugger lists this message for all images on the page. In most cases, the images are either jpg or png files.
If I turn off hotlink protection, the message goes away and the images return.
Is there another URL I should be using for FB images in my hotlink exceptions list?
Referer? If you check these requests in your server's access log then that should tell you precisely what you need to add to your exceptions list.