For some reason (even before I installed this plugin) I was able to get the correct thumbnails to show up on Facebook... BUT only when I had a single picture in a post.
My issue is with posts that have multiple images (examples below). I do get some images to show up... but usually, none are the Featured Image that I've set in my post:
Example 1 (Multiple Pics in Post)
Example 2 (Multiple Pics in post)
When I installed this Plugin, nothing changed when I tried to share them on FB :/