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You can create custom audiences by Facebook id (or email, phone number, etc) https://www.facebook.com/business/help/606443329504150?helpref=faq_content https:/

• downandout 9y ago
  This only applies to people that have installed your Facebook app (not to their friends). In the CA case, that means they would have been able to specifically target a few hundred thousand people, but they didn’t do that because the app had long been deleted before the 2016 election. Facebook actually cares about this issue. The bug bounty I was awarded arose from the fact that you could build a custom audience email list using user_nickname@facebook.com without knowing their actual email address. So you could simply write a bot that visited Facebook.com/profile.php?id=4 and see that it redirected to Facebook.com/zuck, and now knowing that “zuck” is the user nickname corresponding to user ID 4, you could then put him in your custom audience list using zuck@facebook.com. That worked for all 2 billion people on the platform, because at one point Facebook gave everyone an @facebook.com email address that mapped to their account. However, that issue has been fixed and was fixed before the election.