To expand on this: with targeted ads, if I want to find out if you're gay I need to figure out a way to track you across sites that give me that information, or

To expand on this: with targeted ads, if I want to find out if you're gay I need to figure out a way to track you across sites that give me that information, or to specifically buy that data from a company that's already tracking you. With FLoC, anyone can get that data without entering into an expensive agreement. I can get that data as a single person from a personal blog with no cookies. I can get that data directly, I don't need to sign up with another 3rd-party tracking company that might not tell why they're serving my users a specific ad or what data they have about them. And it's all scaleable with no extra cost to my operations. This opens up additional attack vectors that might not exist otherwise: in a small operation it may be a lot easier for me to fingerprint you using your IP address, login information, or other data I have access to and to correlate that to a real-world identity. When that happens, I also have access to all of the information about you from your FLoC categories, for free. On some level, Google is taking a gamble that users will be difficult to fingerprint or identify using FLoC so it won't matter that some sensitive information is leaked. But it's just not the case, users can be fingerprinted. And in scenarios where they are fingerprinted/identified, FLoC provides a much more complete picture of that person based on activity from sites that might not have ever sold that information, that might have never intended to leak information about them in the first place.