All the data gathered by Cambridge Analytica was gathered through the public API though, after users had consented to share it (by logging into a quiz app that requested the permissions). That’s why the API is very locked down now, and the approval process to get any sort of data access is very strict.

The main issue was that they gathered data from people whose profiles were set to be visible only to friends. If someone logged into the quiz and granted permissions, their friends’ data was also accessible via the API.