Instead of doing the automatic sharing on Facebook, try using the Facebook share button on the image page after you submit the image. That way you get a large and clear picture on Facebook.
I noticed that on mine too and disabled that feature because of this. I now manually post links to photos and they look fine. It's a shame that FB won't work with FAA to allow us to use the automated system, that use to be a nice feature.
It looks like Facebook has changed it's methods. The past few days if I do automatic sharing it shows a much larger and sharper image, but it's cropped at the top and bottom.