I know that in order to display best quality images on Facebook, the image should be under 100K. However, when I try to export my images with the "limit file size to 100K" box checked, I get an error message and the image will not export. I know that my images do not look perfectly sharp on Facebook but don't know how to remedy this.
I normally export copies specifying the long side to be 1200 pixels. I usually set the quality to about 70. My images seem to look fine on Facebook. I don't worry about limiting the file size.
I normally set width to 1800px and height to 1200px with jpg quality set to 90. I do not set max file size; they range from 500KB up to 2MB. My images also look fine on Facebook.