So after some research, it appears to be the metadata that Facebook adds to photos that are uploaded (edit: or downloaded for tracking where the image goes) to their platform. I haven't found anything that points towards how to search for the image using the metadata key, but if I was taking an educated guess here, he left something in a photo that he uploaded to Facebook that could be found by using the many programs used to get the fbmd from the images.

This would be similar to using a checksum to identify the correct image. Find the image with that metadata key, find the answer.

So after some research, it appears to be the metadata that Facebook adds to photos that are uploaded to their platform. I haven't found anything that points towards how to search for the image using the metadata key, but if I was taking an educated guess here, he left something in a photo that he uploaded to Facebook that could be found by using the many programs used to get the fbmd from the images.

This would be similar to using a checksum to identify the correct image. Find the image with that metadata key, find the answer.