Facebook founder Mark Zuckerberg has been ordered to testify in a landmark trial in the US over the impact of social media on young people.

Los Angeles County Superior Court Judge Carolyn Kuhl this week rejected the argument his company, Meta Platforms, had made that an in-person appearance was unnecessary.

Her order also applies to Snap boss Evan Spiegel, as well as Adam Mosseri, who leads Meta-owned Instagram.

The trial, expected in January, is among the first to advance from a wave of litigation accusing social media companies of making their apps addictive and enticing to young people despite being aware of mental health and other risks.

Meta did not respond to a request for comment.

Law firm Kirkland & Ellis, which is representing Snap, said the decision did not "bear at all" on the truth of the claims.

It said it looked forward to the opportunity to explain why they believe the "allegations against Snapchat are wrong factually and as a matter of law".

Hundreds of claims brought by parents and school districts were consolidated into one case before the Los Angeles County Superior Court in 2022.

They accuse the companies of having ineffective parental controls and weak safety features, and also say that alerts for "likes" and other responses keep young people tied to the platforms.

Meta and Snap have contested the claims, which are similar to those in a separate but similarly sprawling federal case. TikTok, owned by ByteDance, and YouTube, owned by Alphabet, are also named in the suits.

In seeking to have both cases dismissed, the tech companies have argued that federal law protected them from responsibility for content on their platforms. Under a law passed in the 1990s, they are not liable for what people say or post on their services.

But the Los Angeles judge said the companies must still face claims of negligence and personal injury stemming from the apps' designs.

Lawyers representing young people and their parents argue that the companies decided not to make changes because they were concerned about the hit to the business.

Meta had said Zuckerberg and Mosseri had already submitted to questioning as part of the case and that appearing in person represented "a substantial burden" and would "interfere with business".

But Judge Kuhn wrote that hearing directly from the heads of the company was key to evaluating those claims.

"The testimony of a CEO is uniquely relevant," Judge Kuhn said, as their "knowledge of harms and failure to take available steps to avoid such harms" could help prove negligence.

Beasley Allen, one of the firms law firms leading the litigation against the social media companies, was pleased with the ruling.

"We are eager for trial to force these companies and their executives to answer for the harms they've caused to countless children," it said in a statement.

Social media companies have been facing growing legal and political pressure arising from concerns about the impact of the apps on young people's mental health.

Testifying before Congress on the issues last year, Zuckerberg said his company took the issues seriously and defended the protections it had in place, while distancing it from responsibility.

"The existing body of scientific work has not shown a causal link between using social media and young people having worse mental health," he said.

Instagram started rolling out special "teen accounts" in response to concerns last year.

It updated that system earlier this month, adding a default setting that screens out content guided by a system similar to movie ratings. It also said parents could opt for stricter controls.