The Facebook compression has never been stellar. With the advent of "High Quality" uploads i had thought that it would improve (and to be fair it did at the time) however I think that whilst the quality is perfectly fine for your average selfie... well I'm not happy with it.
Both of the following files are the same, the only difference is that the top photo was uploaded to Facebook and then downloaded again. In both cases the file does look a tiny bit different before Squarespace (my hosting provider) has applied it's sauce to the image but the degradation is minimal/insignificant. The Facebook compression on the other hand is VERY visible.
Illustrations in photo by Alisa Pavia