Facebook's API model used to be the app can see everything you can see. Resulting in fun apps like friendship graphs, interest similarity scores, photo collage generators. Basically, features from a Facebook Labs team that doesn't exist.

Then Cambridge Analytica scooped up all the profile data of all of your friends when you completed a fun Buzzfeed style quiz and that created enough outrage to change it.

Whether Cambridge Analytica was actually able to make any use of this data to "subvert democracy" is very debatable. If you look into it, it's just how they marketed themselves.