Los Angeles, California - CEO Mark Zuckerberg said Wednesday he regretted Meta's slow progress in identifying underage users on Instagram, as he faced stinging criticism at a landmark social media trial over accusations that his company deliberately hooked children.

Asked to comment on complaints from inside the company that not enough was being done to verify whether children under 13 were using the platform, the 41-year-old head of Meta, which also owns Facebook and WhatsApp, said improvements had been made.

But "I always wish that we could have gotten there sooner," he added.

Zuckerberg was the most hotly anticipated witness in the California trial, the first in a series of lawsuits filed by American families against social media platforms.

The trial marked the first time the multibillionaire addressed the safety of his world-dominating platforms directly before a jury and under oath.

Zuckerberg was very reserved at first, an AFP journalist in the courtroom reported. But he grew animated, showing signs of annoyance, shaking his head and waving his hands as he turned toward the jury.

The 12 jurors in Los Angeles heard the increasingly testy testimony as plaintiff lawyer Mark Lanier pressed Zuckerberg on age verification and the tycoon's guiding philosophy for making decisions at the vast social media company he controls.

During questioning by his own lawyers, a more relaxed Zuckerberg described time spent on the app as a "side effect" of a quality experience and often addressed the jurors directly to emphasize his points.

He also stressed his belief that Apple and Google, the companies behind operating systems powering most of the world's smartphones, should arrange for age verification at the handset level instead of leaving it to each app.

"Doing it at the level of the phone is just a lot clearer than having every single app out there have to do this separately," Zuckerberg said. "It would be pretty easy for them."

Zuckerberg faced a barrage of internal emails, including warnings from colleagues that age verification was not fit for purpose and others that seemed to plainly display that encouraging more time spent on Instagram was long a central goal of the company.

The trial is set to last until late March, when the jury will decide whether Meta, as well as Google-owned YouTube, bear responsibility for the mental health problems suffered by Kaley G.M., a 20-year-old California resident who has been a heavy social media user since childhood.

Kaley G.M. started using YouTube at age six, Instagram at nine, then TikTok and Snapchat.

Under-13s are not allowed on Instagram, and Lanier pressed Zuckerberg on the fact that Kaley had easily signed up for the platform.

Zuckerberg was confronted with an internal document that said Instagram had four million users under 13 in 2015, at the time the plaintiff adopted the app, and that 30% of all children age 10 to 12, or "tweens," in the US were users.

Zuckerberg said that "we're in the right place now" when it comes to age verification.

Lanier went on to argue that young people like Kaley were subject to Meta's efforts to increase time spent on its wildly popular apps, despite Zuckerberg having told the US Congress under oath that this was not the case.

Faced with emails displaying internal targets for usage, Zuckerberg admitted that "we used to have goals around time," but that the company's aim was always to "build useful services" that connected people.

Zuckerberg was also read an old email from former head of public policy Nick Clegg that said "the fact that we say we don't allow under-13s on our platform, yet have no way of enforcing it, is just indefensible."

The trial is set to determine whether Google and Meta deliberately designed their platforms to encourage compulsive use among young people, damaging their mental health in the process.

The case is expected to establish a standard for resolving thousands of lawsuits that blame social media for fueling an epidemic of depression, anxiety, eating disorders, and suicide among young people.

TikTok and Snapchat, also named in the complaint, reached settlements with the plaintiff before the trial began.

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