2 points
How is Facebook able to know what site is requesting it? Is it in the referer header, or is it parameters in the javascript/image url?
2 points
There is a referer header sent, but depending on the exact code added to the page, it’s very likely they are loading a snippet of JavaScript that lets them collect other information and trigger their own sending of information to their server.
For example, Google Analytics has javascript added to the page, but loading fonts from Google’s CDN (which many sites do) will rely on the referer.
