Mark Zuckerberg, the boss of Instagram-owner Meta Platforms, appeared in a California court on Wednesday defending his leadership of the company in a landmark legal case over whether social media is addictive for children.

It is Zuckerberg's first appearance before a jury after years of backlash against Meta, which also owns WhatsApp and Facebook, which he co-founded.

Meta's lawyers have argued that the lead plaintiff in this case, known by her initials K.G.M., was harmed by other factors in her life, not by her use of Instagram.

The trial, in which Google's YouTube is also a defendant, is being closely watched for its implications for thousands of similar lawsuits facing social media firms.

TikTok and Snapchat, which had also been named in the lawsuit, settled shortly before the trial was scheduled to begin. Terms of the settlements were not disclosed.

Lawyers for K.G.M. or Kaley, who started using Instagram and YouTube as a child, have accused social media firms of working to addict young users, despite being aware of risks to mental health.

She attended the proceedings on Wednesday, sitting directly across from Zuckerberg, who arrived at court with an entourage of security and associates.

Bereaved parents were also among those in the courtroom to watch the proceedings.

Mark Lanier, representing K.G.M., pressed Zuckerberg about his efforts to get users to spend more time on his platforms, confronting the executive with emails he had written.

In 2015, for example, Zuckerberg told a group of executives that his goals for the year included seeing "time spent increase by 12%" and the "teen trend be reversed."

A year earlier, he had identified time spent as "the most concerning" metric being tracked by the company.

Gesturing to his client, Lanier noted that the emails were written when she was only nine or 10 years old, and already using Instagram.

Zuckerberg admitted that "at an earlier point in the company" he would give executives goals to increase time spent, while also insisting that was no longer how the company operated.

He also said that if something is of value, "people tend to use it more."

Lanier noted that people who are addicted to something also tend to increase their use.

"I don't know what to say to that," Zuckerberg replied. "I think that may be true, but I don't know if that applies here."

In questioning last week, Adam Mosseri, the head of Instagram, challenged the idea of social media addiction, arguing that even 16-hours of Instagram use in a single day did not show an addiction.

The trial is expected to last several weeks. It is also set to include testimony from former Meta employees who have since spoken out about the company's practices.

YouTube boss Neal Mohan was also expected to appear but is no longer being called to testify, the BBC has learned.

The case is one of thousands of similar lawsuits brought by families, state prosecutors, and school districts that are currently winding their way through U.S. courts.

The lawsuits accuse Meta and other social media platforms, including TikTok, Snapchat and YouTube, of functioning in an addictive manner that ultimately has harmed many children.

In one such case, 29 state attorneys general are urging a California federal court to order the platforms to make several changes immediately, before any trial, including requiring Meta to remove all accounts known to belong to users under 13 years of age.

A growing number of countries are restricting social media use among young people. Australia implemented a ban on social media accounts for people under 16 late last year.

The UK, Denmark, France and Spain are considering similar measures.