I dunno, you don’t need to be particularly concerned with security to understand that you do not need a facebook tracking pixel on your ‘paying customer’ UI.
Facebook want data on what actions users took before signing up, which users actually signed up and started paying, and how that relates to revenue. This UI is exactly where they can determine these types of actions.
Whether this actually makes Facebook better at marketing or not is a good question.
Killer mistake of your revenue, or killer mistake of what we'd prefer apps to be?