Want me to ask Zuckerber why he does that to Chevereto? Is just how Facebook works, most likely no one has an idea how to workaround that thing.
Actually... This could be related to the use of the .md version of the image. What I've noticed is that Facebook always do a resample of the shared images so in this case the source image is the small .md image and not the original image. The result is that Facebook does an image resampling using an small image and the results are worst than using a large image.
I will try using the original image, it will get blurry look but maybe it won't be that hard.