Mark Zuckerberg was sharply questioned on the witness stand about whether he and other leaders at Meta Platforms Inc. are aware of the volume of children under age 13 who use Instagram.

During a landmark trial over social media addiction, Zuckerberg described the “very difficult” task of enforcing the platform’s age requirement. He said Meta has introduced some “proactive tools” to try to identify and remove accounts violating the rules.

“There are a set of people — potentially a meaningful number of people — that lie about their age,” Zuckerberg told the jury in Los Angeles Superior Court, noting that it’s a “challenging” problem.

The chief executive officer of Meta and the world’s fifth richest person is the second executive to testify during the trial, which started Feb. 9, and centers on Kaley G.M., a 20-year-old woman who blames Meta’s Instagram and Google’s YouTube for her years of mental health struggles.

He said there have been debates at Meta about “privacy sensitivity” related to asking individuals for their date of birth in order to create an account, something the company ultimately decided to do.

“I think we got to the right place over time,” he said. “I always wish we could have gotten there sooner.”

Kaley, who is also identified in court documents by her initials K.G.M., was present in court for a portion of Zuckerberg’s testimony. She has been absent for much of the trial so far after her lawyer Mark Lanier told jurors it would be traumatic for her to sit through it.

Lanier told Zuckerberg that Kaley had an Instagram account when she was nine years old.

“You expect the nine-year-old to read all of the fine print?” Lanier said.

Meta has long argued that age verification should happen before a user downloads an app — meaning that Apple Inc. and Alphabet Inc.’s Google, which control the world’s most dominant mobile operating systems and app stores, should be responsible for age-gating certain experiences.

Meta, Apple and Google have all lobbied in various U.S. states to get ahead of potential legislation that could determine which companies are ultimately responsible for this type of user protection.

The trial, which is expected to run through the end of March, will serve as a critical test for thousands of other lawsuits that target not only Meta and Google, but also TikTok Inc. and Snap Inc. The latter two companies aren’t participating in the current case because they reached confidential settlements with the woman’s lawyers at the Seattle-based Social Media Victims Law Center shortly before trial.

While the four social media giants have denied wrongdoing and maintain they have installed robust guardrails for young users, they face billions of dollars in potential damages if juries side against them in early trials.

Lanier told jurors at the outset of the trial that he planned to “quiz” Zuckerberg about the company’s goals related to attracting and retaining young users, and how he balances business interests with safety.

In questioning Wednesday, the lawyer confronted Zuckerberg over a 2015 memo in which the CEO outlined the company’s goals for the coming year and said he wanted to “reverse the teen trend” and “increase time spent by 12%.”

Other documents made public years later by an employee-turned-whistleblower showed that Meta also faced declining teen usage on Facebook, its core network, forcing employees to strategize about how to “optimize” its networks for young people. In recent years it has made attracting young adults to Facebook a key focus, tweaking its algorithms to surface more content from outside a user’s network of friends and family — a strategy popularized by TikTok.

Profit versus safety was a central theme in the Feb. 11 testimony of Instagram head Adam Mosseri, who Lanier peppered with questions about the company’s decision to lift its ban on photo filters that replicate the effects of cosmetic surgery. Internal emails showed Mosseri and Zuckerberg supported lifting the ban even after staffers called into question whether the so-called beauty filters would do more harm than good.

Lanier also posed a series of questions to Mosseri about how much the company valued testing and studying the impact of a given product or design choice on users before releasing it to the public. The lawyer highlighted the original motto for Facebook coined by Zuckerberg, “Move fast and break things.”

Meta has been criticized for years for allegedly failing to protect young people online. Internal documents unveiled in 2021 found that employees were aware that Instagram could have negative effects on teens, especially girls. During a Federal Trade Commission antitrust trial in Washington last year, other internal documents showed that Instagram’s automated software systems recommended that child “groomers” connect with minors on the app.

Zuckerberg has previously had to defend his company before Congress. In January 2024, during a congressional hearing over youth safety on social networks, Zuckerberg stood up and apologized to families of children who were victims of sexual exploitation on social media platforms.

The company has made efforts of late to improve its privacy settings for teen users. It debuted so-called teen accounts in late 2024 that automatically restrict content and some interactions on Instagram for kids under 18. It also changed the default content settings on Instagram in October to what it described as “PG-13” for all users under 18, and now restricts some younger teens from streaming live on Instagram.

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With assistance from Kurt Wagner.

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