"We did take action," Zuckerberg answered. "We took down the app, and we demanded that both the app developer and Cambridge Analytica delete and stop using any data that they had. They told us that they did this. In retrospect, it was clearly a mistake to believe them."
Some senators argued the problem is Facebook itself.
"Your business model is to maximize profit over privacy," Blumenthal said.
They said Facebook doesn't make it clear enough how that data is used and how to protect it.
"Is Facebook being safe?" said Sen. Debra Fischer, R-Nebraska.
"Senator, I think Facebook is safe. I use it and my family uses it," Zuckerberg answered. "I think everyone should have control over how data is used. Every time they share something they have control right there about who they share it with."
Zuckerberg's decision to share appeared to pay off for him and the company. After a month-long slump, Facebook stock jumped 4.5 percent Tuesday afternoon, driving his net worth up by nearly $3 billion.
"Nothing in life is free," said Sen. Orrin Hatch, R-Utah. He argued that data collection comes with benefits for users of Facebook and Google.
"These great websites that don't charge for access, they extract value in some other way and there is nothing wrong with that as long as they are upfront about what they are doing," Hatch said.
Zuckerberg revealed that some Facebook executives have been interviewed by special counsel Robert Mueller, who is investigating Russian meddling on the site and elsewhere. This is the 33-year-old executive's first time testifying on Capitol Hill, and his second time will come Wednesday, when he goes before a House committee.
Wired editor-in-chief and CBS News contributor Nick Thompson said he felt Zuckerberg "did OK" and "answered the questions thoroughly and seemed confident."
One thing that stood out to Thompson was Sen. Lindsey Graham, R-South Carolina, question about if Facebook is a monopoly.
"That's the hardest question and Zuckerberg doesn't have an answer because they effectively do have a monopoly," Thompson said. "So Graham's follow-up -- how do we trust you self-regulate if you're a monopoly? -- that's one of the hardest questions."
Thompson said Congress will face a decision about regulation. Thompson predicted there "probably" will be regulation on ads and "maybe" on privacy, but it "could get very intense" if Congress deals with antitrust regulation.