The Hitchhiker's Guide to Facebook Web Tracking with Invisible Pixels and Click IDs

Over the past years, advertisement companies have used a variety of tracking methods to persistently track users across the web. Such tracking methods usually include first and third-party cookies, cookie synchronization, as well as a variety of fingerprinting mechanisms. To complement these tracking approaches, Facebook (FB) (now Meta) recently introduced a new tagging mechanism that attaches a one-time tag as a URL parameter (namely FBCLID) on outgoing to links to other websites. Although such a tag does not seem to have enough information to persistently track users, we demonstrate that despite its ephemeral nature, when combined with FB Pixel, it can aid in persistently monitoring user browsing behavior across i) different websites, ii) different actions on each website, iii) time, i.e., both in the past as well as in the future. We refer to this online monitoring of users FB web tracking.

We show how this tag can be used to match, and thus deanonymize, activities of online users performed in the distant past (even before those users had a FB account) tracked by FB Pixel. In addition, by combining this tag with cookies that have rolling expiration dates, FB can also keep track of users’ browsing activities in the future as well. Our experimental results suggest that more than 20% of the popular websites have adopted this technology, and thus can contribute to this kind of activity tracking on the web. Our longitudinal study shows that this type of user activity tracking can go as back as 2015, or even as 2013 (when the precursor of this technology was first introduced by FB). To put it simply, if a user creates for the first time a FB account today, the platform could match the user’s past web browsing activity, collected in anonymous form, to their newly created FB profile, from as far back as 2015 and continue tracking their activity in the future.