I’m curious to see where this experiment is going 
I agree with you. Personally don’t use facebook but for instance, when I upload videos to my vimeo free account, I fed them frogs in the shiniest possible dresses (x265 10bit). Nevertheless they end up converted to a medium rare 8bit x264 
The WF I described above is mainly for sending people ref visuals and for my blog stuff. Realistically I could compress them quite a bit more and most people would never notice, but again I tend to inject loads of botox-grain in my monster-princesses and their faces melt badly. The backed up edited tifs and downsized jpegs are uncompressed.
I am not using FB anymore, but when I used I made experiments with portrait photos. This is what I found out:
- Downscale to 960 or 2048 px (which one you prefer) and apply sharpening etc. Better to be not so gentle with sharpening, FB’s compression is going to wipe out plenty of detail anyway. For profile and cover pictures there are different guidelines for optimal size.
- 2048 px usually gives better results than 960 px, even in portrait orientation when browser has to downscale the photo. FB’s compression is rather heavy and skin tones take heavy hits in 960 px. Beware especially jpeg compression artifacts on skin - 960 px was prone to these. Rationale here is that jpeg compression algorithm focuses on compressing areas with little to no detail and skin is the one that can get its fair share of compression. Adding subtle amount of noise (detail!) can help
- Upload high quality jpegs and enable “high quality”. My default is 97 , higher only if there are very subtle tones, like shadow areas in BW shots. If I recall it correctly, you only lose if you upload precompressed images like 75 “for web”.
- People say that png can give better result, especially in cases like BW shots with subtle shadow tones. I didn’t really notice any difference.
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I mean I personally tend to save my RT image full, then open the JPG file in RT apply sharpening, sometimes using the tone mapper, and yes I agree, definitely, don’t be gentle with it because it has to stand out against Facebook’s compression massacre that is to follow, then save it as 1200px wide side 90% or 100% compression and Best quality and then upload that. The 90% vs 100% depends on whether I feel like waiting slightly longer for the upload or not 
Definitely 90% or even 80% for blog upload (same size - 1200px wide side) because the quality is pretty good when no further web site based compression is applied.
