Mark Zuckerberg has admitted the misuse of users' data was a "big mistake" as he gave testimony to Senate committees in Washington.
The founder of Facebook is answering questions from senators in light of revelations that Cambridge Analytica used Facebook data to influence US voters.
Mr Zuckerberg said that Facebook will take a more "proactive position" when it comes to dealing with apps in future.
The Facebook boss started two days of questioning by committees in Washington after be
ing called to testify.
It is after UK firm Cambridge Analytica got access to data from Facebook users.
Zuckerberg: ~"I hope what we do with data is not surprising to people." When I explain to people what Facebook does with the data and also how it collects and aggregates data on people, people's jaws are on the floor. Smart, educated people, too. It's all very obscure.— zeynep tufekci (@zeynep) April 10, 2018
The social network has been sending messages to those people who have been affected.
In his opening statement, Mr Zuckerberg took personal responsibility for the scandal.
"We didn't take a broad enough view of our responsibility and that was a big mistake and it was my mistake and I'm sorry.
"I started Facebook, I run it and I'm responsible for what happens here."