According to the Guardian and the New York Times, a Cambridge University researcher named Aleksandr Kogan produced an innocuous-looking personality testing app for Facebook whose real purpose was to identify the sorts of marketing pitches one might be susceptible to — ones that played to people's anxieties, for example, or alternatively to their sentiments. He then gathered data not just from the roughly 270,000 people who used the app, but from tens of millions of their Facebook friends, all without the friends' knowledge or consent, according to the news articles.
The story gets worse, however. Kogan reportedly turned over the Facebook data he had harvested to a political consulting firm, Cambridge Analytica, to help it build profiles that it could use to sway voters on a massive scale. The messages could be tailored precisely to the weaknesses of a narrow group of voters, and each pitch could be confined to a single group to avoid putting off voters with different sensibilities.
As Christopher Wylie, who worked at Cambridge Analytica from its founding until 2014, told the Guardian, "We would know what kinds of messages you would be susceptible to, and where you're going to consume that. And then how many times do we need to touch you with that in order to change how you think about something."