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.