Cambridge Analytica and Facebook

Love it or hate it, Facebook is one of the biggest success stories of the past century. Billions of people have profiles, and for many users it's the only source of online news or social interaction. And Facebook collects as much information as possible on each of these users, ostensibly to better target advertising campaigns. So what happens when someone starts using this vast collection of data for something else? What is Cambridge Analytica, and how deep does this rabbit hole go?