The alleged hackers improperly used a Facebook feature that helped them take control of users’ internet browsers and gave them access to private information about Facebook users and their private friends’ lists, Facebook alleged in a lawsuit filed in Northern California on Friday.

…Once users connected their Facebook and other social media accounts they were asked to install what Facebook described as “malicious browser extensions” that essentially allowed the alleged hackers to pose as the affected users online.

Ukrainian hackers used quizzes to access private Facebook data, Facebook says – CNN

hmmmm