De-anonymizing Facebook's app-scoped ids
Facebook has taken plenty of criticism for privacy problems over the years, and it's invested plenty of resources in responding. One specific problem early on was that third party apps could combine their data to create deeper user profiles for tracking and analysis. If one app couldn't get permission to see a Facebook user's friends, for example, it might quietly partner with another app that did instead of trying harder to get official permission. Facebook fixed this in their v2.0 API by giving each app its own set of app-scoped user ids.