The facebook tags use this icon: , which is a image of size 18px x 18px.
<img> is starched to size 18px x 16px:
<img width="18" height="16" ... >
This results in all icons slightly blurred, distorted and disproportional.
There are many icons available in suitable sizes, for example Facebook's favicon: (which also appears sharper).
This is extremely important.