- data you give it (birthday, religion, likes)
- data it learns about you. (ad tracking, habits, demographics)
facebook built a developer tool that let app developers ASK USERS for "data you gave it"
users clicked YES when prompted with the question, GIVING the app developers the data. Cambridge Analytica NEVER got to touch "data facebook learned about you."
the way the developer tools were designed, when you gave the app access to your facebook account, it could look at things YOUR FRIENDS had made avaliable to YOU. The app could act as you, and see what you see. You can never see "data facebook learned about your friends."
The issue is, people couldnt trust their friends not to click "SHARE DATA." The issue is IF its inappropriate for apps I install to see data shared with me by my friends. Once facebook learned this ability was being misused, they shut it down, back in 2014.