Mark Zuckerberg has been criticised by US politicians for his company's failure to remove terrorist content, its plans to introduce its own cryptocurrency and perceived bias over what views it promotes.

The questions came as Mr Zuckerberg testified in front of the House Financial Services Committee, about Facebook's plans to create a global digital currency that would be available to all users of its services.

But criticisms focused not just on that plan, but all of the various ways that Facebook conducts business around the world. Lawmakers asked about whether Facebook encouraged discrimination, for instance, as well as pointing out flaws in its content moderation techniques.

During the hearings, Mr Zuckerberg was informed that videos of mass killings such as the Christchurch mosque shooting was still being hosted to watch by Facebook. He agreed that the company was "doing its best" to try and take such videos down, but suggested it would never be possible to catch them all.

Mr Zuckerberg's hearing was advertised as focusing on Libra, the cryptocurrency that regulators fear could undermine government's power and traditional money. In recent months, the plan has come under scrutiny from lawmakers and has lost many of its biggest supporters.

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Several high-profile companies that had signed on as partners in Facebook's governing association for Libra have recently bailed, spelling a potentially rough road for the project. But many experts don't believe it's doomed.

Zuckerberg, in written testimony prepared for the hearing, aimed to reassure lawmakers that his company won't try to evade financial regulators as it readies Libra.

Facebook "will not be a part of launching the Libra payments system anywhere in the world unless all U.S. regulators approve it," he said. That's a stronger statement than Facebook official David Marcus made to Congress in July, when he said the company will not activate Libra until it has "fully addressed regulatory concerns and received appropriate approvals." Marcus leads the Libra project.

Zuckerberg is striving to defend Libra and alleviate concerns that the currency could sidestep regulators. Analysts say Libra could avoid regulation and launch in countries where it's not getting pushback, but this doesn't appear to be Facebook's intention.

Instead, Zuckerberg is pushing an optimistic vision of Libra and what it could mean for people around the world who don't have access to bank accounts.

There is concern among regulators that the massive reserve created with money used to buy the new currency could supplant the Fed and destabilize the global financial system, and that consumers could be hurt by Libra losses.

Zuckerberg also played the China card in his remarks, urging regulators to act quickly "While we debate these issues, the rest of the world isn't waiting. China is moving quickly to launch similar ideas in the coming months," he said.

The Facebook CEO also has cited competition from China as a compelling reason against breaking up the company.

The Justice Department, the Federal Trade Commission and the House Judiciary antitrust subcommittee are all conducting investigations of Facebook and the other huge tech companies amid accusations of abuse of their market power to crush competition.

Sen. Elizabeth Warren, a leading Democratic presidential candidate, has advocated breaking up Facebook and other tech behemoths. She recently ran a fake political ad on Facebook taking aim at Zuckerberg to protest the company's policy of not fact-checking politicians' speech or ads in the same way it enlists outside parties to fact-check news stories and other posts.

In a major speech last week at Georgetown University, Zuckerberg defended the company's refusal to take down content from its platform it considers newsworthy "even if it goes against our standards."

Facebook, Google and Twitter are trying to oversee internet content while also avoiding infringing on First Amendment rights. The pendulum has swung recently toward restricting hateful speech that could spawn violence.