As reported by various media and confirmed by the company itself in various press releases, the privacy of millions of Facebook user accounts worldwide has been compromised. This information leak has been caused by the use of social network data by Cambridge Analytica. Despite the fact that in 2015 this company formally certified the erasure of all the data acquired in this way, Facebook has continued to investigate the case and published several press releases on the impact and scope of the incident based on its estimates, as well as some measures to be adopted.
[Update 25/07/2019]: Facebook and the Federal Trade Commission (FTC) have reached an agreement that Facebook should review the way it treats its users' data and impose a fine of $5 billion, equivalent to 9% of its revenue in 2018. It also accepts the imposition of establishing a council (made up of an independent body) to supervise the use of its users' data.
On the other hand, the SEC (Securities and Exchange Commission) has announced that it will fine Facebook $100 million for misleading and inaccurate statements to investors about the misuse of their users' data. Facebook has agreed to pay this fine.