Facebook will investigate "tens of thousands" of apps to discover if any other companies have accessed data in a similar way to Cambridge Analytica (CA), Mark Zuckerberg has told US politicians.
The social network says it is in the process of letting up to 87 million users know that their information may have been accessed by CA and, in a packed room on Capitol Hill, Mr Zuckerberg repeated his admission that the company "didn't do enough" to stop its tools "being used for harm".
He also said the company was in an "arms race" with Russia, warning there were people in the country tasked with finding ways to breach Facebook's systems.
After a torrid few weeks for the company's share price investors rewarded Mr Zuckerberg for appearing on the Hill.
Facebook stocks rose 4.5% on Tuesday, which is said to have swelled the entrepreneur's fortune by several billion dollars.
Speaking to the Senate Judiciary and Commerce Committees, the tech entrepreneur apologised that Facebook had not taken a "broad enough view" of its responsibility for people's public information.
"It was my mistake, and I'm sorry. I started Facebook, I run it, and I'm responsible for what happens here," he said, in words from a prepared statement.
Rep. Greg Walden, chair of the Energy and Commerce Committee, called Zuckerberg an embodiment of the American Dream and one of the modern era's "greatest entrepreneurs".
However, he expressed concern that Facebook had grown too quickly. Quoting the company's early motto to "move fast and break things", Rep. Walden asked whether the company had "moved too fast and broken too many things".