Mark Zuckerberg testified that it’s “very difficult” to enforce Instagram’s age limits and downplayed how much teen users do for the company’s business during a landmark trial over social media addiction.

The chief executive officer of Meta Platforms Inc was sharply questioned on the witness stand Wednesday about the company’s efforts to attract and engage teens, and whether it adequately policed accounts belonging to children under 13, despite rules barring them from using the app.

Zuckerberg said Meta has introduced some “proactive tools” to try to identify and remove accounts used by children under 13, but called it a “challenging” problem.

“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.

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The Facebook founder, the world’s fifth richest person, was 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 fielded questions from lawyers for roughly six hours, and is not expected to return to the stand again.

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 and Snap Inc.

While the 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.

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 in his opening statement that it would be traumatic for her to sit through it.

Zuckerberg, dressed in a dark blue suit and gray tie, at times appeared visibly uncomfortable and frustrated, particularly when Lanier suggested that Meta’s goals were focused on maximizing time spent on its apps.

Under questioning from Meta’s lawyer, Zuckerberg testified that while it’s true the company wants teens to use its services, that cohort isn’t a meaningful revenue driver. Teens account for just 1 percent of the company’s revenue, he said. Meta makes almost all its revenue from advertising.

“Most teens don’t have that much disposable income,” Zuckerberg said. “In terms of our business, I don’t think it’s a meaningful thing in the near term.”

The company adopted a policy that new users must enter their date of birth in order to create an account after internal debates at Meta about “privacy sensitivity,” Zuckerberg said.

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

Lanier told Zuckerberg that Kaley had an Instagram account when she was nine years old — a time during which the company was seeking to increase how much time users spent on its platforms, the plaintiff’s lawyer said, referencing internal company documents.

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

At one point, Lanier’s team unfurled a large banner showcasing a collection of thousands of selfies Kaley had posted to her Instagram account. The display stretched across much of the courtroom and Lanier urged Zuckerberg to view the images, as he explained that she had spent her adolescence posting copious amounts of content to the platform.

Lanier also presented a series of emails, slides and internal messages from Meta employees spanning several years that suggested the company saw the young demographic as key to its platforms’ long-term success. The documents presented a nuanced and in-depth understanding of how to reach different age groups, spanning from pre-teens, or “tweens,” to older teenagers.

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The documents also showed that some Meta employees had concerns about company policies surrounding child safety. Lanier pointed Zuckerberg to an email from Nick Clegg, who was then Meta’s top policy executive, stating that age limits were unenforced, making it “difficult to claim we are doing all we can.”

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 dominant mobile operating systems and app stores, should be responsible for age-gating certain experiences.

Zuckerberg said Wednesday that several companies lack reliable ways to verify a young user’s age, particularly children without a driver’s license. He argued that having phone makers bear more of that responsibility would be a “very wise and simple way to do it.”

Meta, Apple and Google have all lobbied in various US states to get ahead of potential legislation that could force them to take responsibility for age enforcement.

Documents made public in 2021 by an employee-turned-whistleblower showed that Meta 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.

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.

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.