Man, i hate finally admitting this on HN, but I spent a bunch of years as an FB employee, as a data scientist (on ads). I can speak with complete sincerity and

Man, i hate finally admitting this on HN, but I spent a bunch of years as an FB employee, as a data scientist (on ads). I can speak with complete sincerity and say that shadow profiles were not a thing, and were never a thing during or before my time (before, I can't be 100% certain, but I schlepped through the repos and never found examples of same). What generally happens is that you pick a userid (maybe zero for arguments sake), and everyone who doesn't match to an FB userid gets that number. It didn't make it impossible to build an individual profile, but it made it much, much, much more difficult and I never saw anyone do it. I left in 2018, and would be massively surprised if anyone had built this since. Now, it is entirely possible that not everyone was as rigorous as removing userid=0 (for example) and so some FB data probably counts them, and they may be in some of the clustering models but the notion that they have profiles indexed by browser/device id is completely false (for ads at least, some of the crap they did for PYMK was insane).