That's really interesting. The way I understand tracking cookies is that websites the user visits needs to have some snippet from Facebook on them for that snippet to be able to read the cookie and update it right? And since most sites have the Share on facebook button that's potentially how the tracking cookie is picked up by Facebook.

Yup, that's exactly the behaviour.

Also a reason I deleted cookies, history and saved credentials and just use incognito for FB nowadays. It also stops the habitual browsing that gets you hooked.