A link that is shared on facebook gets an additional fbclid query parameter (interesting that google has it's equivalent glclid query parameter for links from google adds).I think that it is easy to get rid of this query parameter, when want to use such a link. (just adding, as I don't see that mentioned in the pdf)

I think that the purpose of this query param is to act as a substitute for cross site cookies: when you click on some link then the original url is further transfered as the http referrer header of the http request, if they can get hold of the logs of some affiliated site, then they can possibly track the flow of these fbclid attributes.

Interesting to observe, how every tiny detail is getting used for tracking purposes. I guess that browsers won't get rid of any specific query parameters when the url is passed as the referrer header, as that would be a violation of the protocol. However it may be possible to write a browser plug-in that does so.