Web pixels are one of the predominant techniques for tracking conversions and user behavior on the Web. The integration of Meta Pixel (the most widely used tracking pixel) into a website enables Meta to collect sensitive information about the website's visitors and match it with their Facebook or Instagram profiles. In addition to detailed navigation history, Meta Pixel also collects personally identifiable information (PII) entered by visitors in online forms present on the website, such as emails and phone numbers.

Over the past years, advertisement companies have used various 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. Facebook (FB) (now Meta) recently introduced a new tagging mechanism that attaches a one-time tag as a URL parameter (namely FBCLID) on outgoing 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 as FB web tracking.