By the way can you add an option that supports custom field in the next update? Say, a new option that lets users manually input the custom field they want to pull the image from. This would be great for those who use a theme that can only add featured images through custom fields. Just a suggestion though.
WordPress has a built-in function for featured images, which is supported by Add Link to Facebook, so I see no reason to support non-standard ways to add featured images. IMHO it is better to ask your theme developer to support the built-in WordPress function.