Remember, services like Facebook are free because you’re the product being sold

If you “liked” it or commented on it, then the news would further spread in the same way to your friends. If you did, that would be logged. The kinds of news you like, share or comment on constitutes one small data point among thousands that Facebook knows about you. They gather data not only from the way in which you interact with Facebook directly, but also by tracking you across the web thanks to “persistent cookies”. They can use these data points to accurately predict your hobbies, age, gender, religion, sexual orientation, marital status, ethnicity, education, approximate income and political views.

But profiles can be used for much more than predicting that one user may be usefully targeted with an ad for designer shoes, while another should be shown an ad for baby formula. They can also be used to predict the kinds of political messaging that may sway their views, and how likely they are to be a useful cog that will re-share a piece of political propaganda (or in the current parlance, “fake news”). And that brings us back to Cambridge Analytica.

Here’s how to see which apps have access to your Facebook data – and cut them off

Cambridge Analytica hired an academic to create one of the ubiquitous “free personality quizzes” that pop up on Facebook from time to time. Anyone who took that quiz willingly gave the app developer not only a remarkable insight into their own psychological state, but also to all of their own information on Facebook, to their list of friends and, in turn, a significant amount of their personal data. All told, this app managed to gather the information of tens of millions of people, even though only 250,000 took the actual quiz. Under Facebook’s terms of service at the time, this secondary data could only be used for “improving the user experience” and could not be resold.