Facebook has a large network of adtech partners who integrate with Facebooks advertising APIs. This used to be FBX, then they replaced it custom audiences. I u

Facebook has a large network of adtech partners who integrate with Facebooks advertising APIs. This used to be FBX, then they replaced it custom audiences. I used to work at one of these firms and AFAIK, I worked in sales, Facebook didn't exchange any data with us. I believe with FBX was implemented using tokens but this changed with custom audiences. What we could do was match email addresses from our clients, think newsletter list, and get custom audience segment to advertise towards on Facebook. I seem to recall that we needed at least 1000 emails and we could see how many of them matched a Facebook user. A possible covert channel of confidentiality breach could be that you feed it 999 bogus emails and one target email to find out if a particular email matched a Facebook user. But then I don't think you would've been able to target ads to that small of an audience. If there is a possibility to get data, my guess is that it would be through inference using this kind of covert channel.