- Protecting the platform - 

"It's clear now we didn't do enough," Zuckerberg said on the protection of private user data and to prevent the hijacking of data on millions by Cambridge Analytica.

Zuckerberg said Facebook was built as "an idealistic and optimistic company" to help people connect but failed "to prevent these tools from being used for harm... that goes for fake news, for foreign interference in elections, and hate speech, as well as developers and data privacy."

He said that by the end of the year Facebook would have 20,000 people working on security and content review and would also step up use of artificial intelligence to weed out fake accounts and inappropriate content.

Read also: Facebook to offer 'bounty' for reporting data abuse

- Regulation - 

Zuckerberg said regulation of social media companies is inevitable, but warned that rules could also hamper the industry's growth.

"The internet is growing in importance around the world in people's lives, and I think that it is inevitable that there will need to be some regulation," he told lawmakers.

"But I think you have to be careful about putting regulation in place. A lot of times regulations put in place rules that a company that is larger, that has resources like ours, can easily comply with, but that might be more difficult for a smaller startup company." 

Zuckerberg said the EU's General Data Protection Regulation (GDPR) to come into effect on May 25 was more stringent than what was currently in place at Facebook and suggested it could serve as a rough model for US rules in the future.

Facebook is implementing the GDPR standards for European users next month, and some of its rules will be extended to US and other users later, he confirmed.

"The GDPR requires us to do a few more things and we are going to extend that to the world," he said.

- Facebook model - 

Zuckerberg maintained that Facebook users deserve protection of private data but appeared to argue that its controls make it possible to determine how information is shared.

He claimed that "there's a very common misperception... that we sell data to advertisers," adding that "we do not sell data to advertisers. We don't sell data to anyone."

But he maintained that advertising enables Facebook to offer a free service and that targeted ads based on user categories were more acceptable to users, even if they could opt out.

Zuckerberg also said the company believed in an ad-supported business model, but appeared to leave open the possibility of a paid version.

"There will always be a version of Facebook that is free," Zuckerberg told the hearing.

Read also: Alibaba’s Jack Ma challenges Mark Zuckerberg to ‘fix’ Facebook

- Russian manipulation - 

The 33-year-old CEO said Facebook was in a constant struggle to guard against Russian manipulation of the Facebook platform to influence elections in the US and elsewhere.

"There are people in Russia whose job it is to try to exploit our systems and other internet systems and other systems as well," he said. 

"So this is an arms race. They're going to keep getting better and we need to invest in getting better at this too."

Zuckerberg has previously acknowledged the social network failed to do enough to prevent the spread of disinformation during the last US presidential race.

"One of my greatest regrets in running the company is that we were slow in identifying the Russian information operations in 2016," he said.

"We expected them to do a number of more traditional cyber attacks, which we did identify and notify the campaigns that they were trying to hack into them. But we were slow at identifying the type of -- of new information operations."

He added that Facebook is cooperating with the special counsel investigation into Russian interference in the 2016 election.

"Our work with the special counsel is confidential. I want to make sure in an open session I don't reveal something that's confidential," he said.

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