Yes, it is. Facebook only honours PNG files that are under 1 megabyte. Anything larger than that, it automatically converts it to a Jpeg file, and NOT kindly. It seems to apply nasty compression to it.
Therefore, you're much better to save it as a Jpeg file yourself, if you see that your PNG is going to be over 1MB.
When saving Jpegs for Facebook, I use 90 on the 0-100 quality scale, or 11 on the 0-12 scale.