Meta Chief Executive Mark Zuckerberg stood up to apologise to families Wednesday at a dramatic US senate hearing in which lawmakers grilled tech chiefs over the dangers that children face on social media platforms.

Tech bosses faced massive political anger from lawmakers who accused them of not doing enough to check the dangerous impact of social media on children, including the threat of sexual predators and teen suicides.

Prodded by Republican Senator Josh Hawley, Zuckerberg stood up, turned to families who held up pictures of children allegedly harmed by social media, and addressed them.

"I'm sorry for everything you have all been through," said the Facebook founder. "No one should go through the things that your families have suffered."

Besides Zuckerberg, chief executives of TikTok, X, Discord and Snap also faced hostile US lawmakers in a session titled "Big Tech and the Online Child Sexual Exploitation Crisis."

"Mister Zuckerberg, you and the companies before us, I know you don't mean it to be so, but you have blood on your hands. You have a product that's killing people," Senator Lindsey Graham told the chief executives.

"We work hard to provide parents and teens support and controls to reduce potential harms. Keeping young people safe online has been a challenge since the internet began and as criminals evolve their tactics, we have to evolve our defenses too," Zuckerberg told the committee in his opening statement.

He added that, according to research, "on balance" social media was not harmful to the mental health of young people.

"I don't think it makes any sense," said Senator Dick Durbin, who chaired the meeting. "There isn't a parent in this room who's had a child...(who) hasn't changed right in front of (their) eyes" because of an "emotional experience" on social media, he said.

Ahead of their testimony, Meta and X, formerly Twitter, announced new steps to check the impact on young social media users.

Meta, which owns Facebook and Instagram, said it would block direct messages sent to young teens by strangers. By default, teens under age 16 can now only be messaged or added to group chats by people they already follow or are connected to.

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