John Wilander, principal engineer on Intelligent Tracking Prevention in Safari, opened an issue on the WICG FLoC github pointing out how FLoC can be used to create cross-site tracking. What this would mean is that the purported privacy benefits of FLoC would be moot. In fact, it would make it easier to track a user.
To take this to the crowd metaphor: Before the pandemic and some time back, I attended a Mew concert, a Ghost concert, Disney on Ice, and a Def Leppard concert. At each of those events I was part of a large crowd. But I bet you I was the only one to attend all four.
Indeed, de-anonymizing users is a huge problem. 99% of Americans can be identified by 15 demographic attributes. As part of this research, they created a tool using just three data points - ZIP code, gender, and birth date - which has an 83% chance of identification.
We aren’t as anonymous as we’d like to think because we’re all wonderful and special little snowflakes.