My Facebook pictures seem to be losing quality. I compared one of them with an old screenshot on Online Image Diff and there was a lot of difference. Is there a way to get my original images back?
Facebook never stores your original image (and, in some cases, never even sees your original image -- the FB uploader compresses overly-large images before even sending them to the Facebook servers).
Since Facebook doesn't have your original image, you can't get your original image back from Facebook.
When uploading to Facebook, make sure you've exported the photos resized to 2048px at the longest edge (most photoeditors will allow you to just set that), and jpeg quality to about 60%. Then when you upload to Facebook make sure you hit the high quality option and start uploading. There will still be some compression but it won't be as gnarly. Most of the quality compression will be done by the photo editor and not by Facebook when doing it this way, which preserves you some quality. Good luck!