> they very explicitly don't let you upload any data unnecessary to find matches.

Is that supposed to be reassuring in some way? I don't understand how that really makes the practice OK.

I'm not saying it's okay, but I'm also pointing out that the system doesn't work that way. Data brokers would not be comfortable handing over all of the data to FB if they thought FB was just going to steal it, and FB wouldn't trust gobs of unprocessed data from third parties ("You are now a 16 year old girl who likes Taylor Swift because there was a typo in the matched email address").

So in FB's documentation, they make it clear that they take the hash of your chosen identifier information (email, ID, etc) and just return the number of hashes that get matched. https://www.facebook.com/business/help/112061095610075?id=24...

They don't tell you who gets matched, and they don't give you a way of providing any other data because they don't trust it.

> Data brokers would not be comfortable handing over all of the data to FB if they thought FB was just going to steal it

... Oh, like the data brokers stole the data from me? sigh I hate adtech.