If you're asking why the image gets blurry if someone makes it smaller, consider:
Your .afdesign document specifies a DPI of 72 pixels per inch.
The .png you showed us of the Facebook icon is 80px by 80px, and just over an inch square. That's not very many pixels.
So my first question would be, how do they make it smaller? I can think of two ways they could do that:
Reduce the number of pixels. If they make it 36px by 36px it will be about 1/2 inch by 1/2 inch. But it has 1/4 of the pixels, and unless if it's viewed at any zoom > 100% yes, it will be blurry because there's not much data there.
Keep the same number of pixels, but make it 144dpi. Then it will also be about 1/2 inch by 1/2 inch, but at its native size it will be sharper than what they started with (more pixels in a smaller area), and they could zoom it to 200% and still have the sharpness of your original.
So I think the real questions are: What are you trying to do? What are the users going to do? And what constraints do you really have on what you need to produce?
Also, could you give them a vector image? Or does it have to be raster?