After days of silence, Facebook CEO Mark Zuckerberg has responded to the controversy over the 2014 leak of private Facebook user data to a firm that went on to do political consulting work for the Donald Trump campaign in 2016.

Cambridge Analytica got the data by paying a psychology professor, Aleksandr Kogan, to create a Facebook personality quiz that harvested data not only about its own users but also about users' friends. Kogan amassed data from around 50 million users and turned it over to Cambridge.

Zuckerberg says that when Facebook learned about this transfer in 2015, it got Kogan and Cambridge to certify that they had deleted the data. But media reports this weekend suggested that Cambridge had lied and retained the data throughout the 2016 presidential campaign.

"This was a breach of trust between Kogan, Cambridge Analytica, and Facebook. But it was also a breach of trust between Facebook and the people who share their data with us and expect us to protect it," Zuckerberg said.

Zuckerberg says Facebook changed its rules back in 2014 to prevent third parties from harvesting data the way Kogan did. But he says Facebook is now going to take additional steps to make sure data harvested four years ago isn't being misused today.

"We will investigate all apps that had access to large amounts of information," Zuckerberg wrote. He promised a "full audit of any app with suspicious activity" and said any developer who refused an audit would be banned from the platform.

Meanwhile, Facebook is tightening its rules for apps accessing user information. If an app goes unused for three months, it will lose access to user data. Apps will initially only get access to users' name, profile photo, and email address.

Finally, Facebook is going to place a tool for managing privacy settings at the top of the news feed. These options are already available in privacy settings, but not everyone knows about the option. Zuckerberg hopes that putting the settings at the top of the news feed will make sure every Facebook user sees it.