Facebook is appalling for image quality. Always has been, and there's nothing you can do about it other than link to somewhere else. Really annoying because i like to share photos with friends after holidays, and while they can download mine at full quality, i can only see their ~600px wide images on Facebook - and when you ask them to email them, they say "why, they're already on Facebook". Grrr!
I noe tend to load one or two onto Facebook, but then put a link to my proper gallery for the rest. Means people who use Facebook still notice something new, but get to see nice quality ones elsewhere.
David