@edpc Ok, so I asked my husband how he'd answer your post. This has been his career for the last 15+ years and is considered an expert in the field of data science. He doesn't generally worry about many things, but he is actually terrified and very angry about how ignorant we are about these issues.
The analogy he uses, is that by the time we are in high school, we generally understand why and elephant and a rock simultaneously falling from a cliff hit the ground at the same time. Conversely, very few people are taught the basic probability/statistics/data analysis so they can even understand the Monty Hall Problem.
The main thing they harvested from FB was not the silly quiz; it was the FB profile itself that is valuable.
From looking at a FB page for 2 minutes, my husband can tell:
My age
Political affiliation
Kids
Kids school
Religion
Which conspiracy theories I'll believe
Hot button issues
How to frame them
Celebrities I'd listen to
Weak links in friends to convince me
Apps on my phone, so how to hack my devices
How to build a consensus among my friends to convince me
People are much more easily led than we'd like to think.
When you read the article about the Cambridge A execs bragging, they discuss using exactly this information, in addition to bribing and blackmailing officials, etc., etc. The Cambridge A people discuss working to craft news stories with troll factories like the Russian one recently profiled.
All of those quizzes you take and likes you make actually are part of a bigger way to build a profile about you.
For example, "Find your stripper name- select the month you were born for the first name, the day you were born for your last name..." You just gave away your birthdate, even if you think you kept that data private.
All of those little type of games and quizzes, put together, can create a fairly complex and accurate profile of you including some very private information.
They can even help give away common questions on 'forgot password' authentication quizzes for banks and credit cards.
Take what one can learn about you from a few minute glance at your profile and multiply it by 1000 based on all the information that can be derived from other activities.