They abstract most of it away. Marketers can set specific targeting criteria and can get a high level estimate of how an ad will perform. Beyond that it's track

They abstract most of it away. Marketers can set specific targeting criteria and can get a high level estimate of how an ad will perform. Beyond that it's tracking impressions, clicks, split testing, etc. The app API was different until 2013(?). App developers only needed a single user's permission to access all of that user's friends' information. Both Obama and Cambridge Analytica came out of that period. Now, users can only authorize the release of their own information.