The question hit on an issue that loomed over the hearings this week: Do internet users really know everything that Facebook knows about them?
Zuckerberg is admitting something many suspected, and in doing so causes Facebook a serious problem.
Facebook tracks users even when they're logged off, and Zuckerberg said that's for either security or advertising reasons.
Zuckerberg said the EU's General Data Protection Regulation (GDPR) to come into effect on May 25 was more stringent than what was now in place at Facebook and suggested it could serve as a rough model for USA rules in the future.
"Congressman, we don't sell people's data", Zuckerberg said.
That data comes from a range of sources, said Nate Cardozo, senior staff attorney at the Electronic Frontier Foundation.More news: The Answer Trump Needs in Syria
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The EU's General Data Protection Regulation, which goes into effect on May 25, also will require companies to tell users what information they are using and why they're using it, how long they will store it and who else will be able to see it. But, as you say, we've had the ability to download your information for years now.
The US-based Facebook last week admitted that 5.62 lakh people in India were "potentially affected" by the global data leak, involving CA. Meanwhile, as the hearing progressed, Recode published a new poll which highlights which tech companies people distrusts the most with their data. "And how much you give away in modern America in the name of, quote, connecting people around the world".
It's time to take measures into your own hands to make sure you see the news you want to see, when you want to see it.
GETTYWill Facebook launch a new paid-for service? The congressional record is open for another two weeks for lawmakers to submit questions, a Facebook source said, after which Facebook will have several more days to respond. In the process, the company said it had amassed contact information from users and matched it against existing user profiles on the social network.
According to Zuckerberg, the firm had "considered it a closed case" after it first took action against Cambridge Analytica and "demanded" both they and the app developer delete the data.
But the issue with Facebook lately is that third parties might be getting your data via your friends when you haven't opted in, and that's not okay.
Under mounting pressure over the hijacking of its user data by a British political consultant, Mr Zuckerberg reiterated his apology for the historic breach, before being grilled over how Facebook collects and protects people's personal information. He cited figures from the Federal Drug Administration indicating there are 35,000 online pharmacies accessible through Facebook, and that "96 percent are operating illegally".