Cambridge Analytica weren’t doing rocket science or hacking. They didn’t get your social security number, your phone number or your browser history.
They built a few ad-targetting criteria with such low precision and accuracy any marketing person would be ashamed of the numbers.
That imprecision played to their ends. Remember the goal was to be disruptive, so although they were using ad-targetting technology, having built-in misses was actually a boon.
What they did was:
1. Get people to fill out a survey,
2. Collected their facebook profile data with consent,
3. Collected uninteresting public profile data about their friends like public group memberships,
Then they used some machine learning systems to:
a- Build an ad-targeting profile of the survey respondents using #1,
b- Build the same ad-targeting profile using #2, but “back-train” it by correlating #1 and (a),
c- Use (b) on that uninteresting public stuff in #3.
They weren’t trying to pin down anyones’ precise political alignment or belief system, they were looking for broad strokes: Watches Colbert, Watches Hannity; Loves Guns, Loathes Guns.