I am trying to create a nice looking image for a Facebook cover image for my company. I created the file in Illustrator, then saved it as a 24 bit .png file. When I post it to FB it looks poor quality with lots of "lossiness." I can't figure out what I did wrong--I thought you didn't lose image quality when saving as a .png file.
that looks like facebook aftermath (running your pixels through a pixel grinder)
might try targeting/saving/uploading at their largest recommended pixel dimensions
otherwise i am not sure what else you can do...it's the nature of the beast
I have to agree, when uploading to facebook for the cover always crop your image to 851 x 315px