It could be because you don’t have any actual images on the page (I do see a video though)…..But I’m not really sure how it all works though. It really just depends on what those social sites look for when creating a thumbnail/image.
I was going to mention adding a featured image, but I’m not sure what good that would do since it probably won’t be visible to those sites when linking directly.
Anyone else out there know how to do this?