The question of addiction is central to the trial, in which plaintiffs seek to hold social media companies responsible for harms allegedly caused to children. Meta Platforms, Instagram's parent company, and Google's YouTube remain as defendants after TikTok and Snap settled earlier.

What is the case about?

The case centres on a 20-year-old plaintiff identified only by her initials, "KGM", whose lawsuit has been chosen as a bellwether trial - a test case that could influence thousands of similar claims.

Mosseri, who has led Instagram since 2018, said it is important to distinguish between "clinical addiction" and what he described as "problematic use". During cross-examination, the plaintiffs' lawyer, Mark Lanier, read out comments Mosseri made in a podcast interview in which he used the term "addiction" to describe social media. Mosseri said he had likely used the word "too casually", as many people do.

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He added that he was not a medical expert and was not qualified to comment on clinical addiction, but said someone close to him had experienced a serious addiction, which made him "careful with my words".

Not clinical addiction, but

Mosseri said he and colleagues prefer the term "problematic use" to describe users spending more time on Instagram than they feel comfortable with. "That definitely happens," he said.

He also told the court that it was not in Meta's long-term interests to prioritise profit at the expense of people's wellbeing.

A tense exchange in court focused on Instagram's cosmetic filters, which can alter users' appearances and were argued to encourage unrealistic beauty standards and even plastic surgery. Mosseri defended the platform's approach, saying the company aims to "be as safe as possible but also censor as little as possible".

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The discussion around body dysmorphia and cosmetic filters appeared to upset some members of the public gallery, including parents who had lost children. The judge reminded the audience not to show any sign of agreement or disagreement with the testimony, describing such reactions as "improper".

Profit targets?

During cross-examination, Lanier suggested Meta was targeting teenagers for profit. Mosseri disputed this, stating that Instagram earns "less money from teens than from any other demographic" because younger users click fewer ads and have limited disposable income.

Lanier countered by citing research suggesting that people who join social media at a young age are more likely to remain on the platform for longer, making them valuable for long-term revenue.

Mosseri said: "Often people try to frame things as you either prioritise safety or you prioritise revenue. It's really hard to imagine any instance where prioritising safety isn't good for revenue."

Meta chief executive Mark Zuckerberg is expected to give evidence next week.

Instagram has introduced numerous safety features aimed at young users in recent years, but critics say the measures are insufficient. A report last year found that accounts set up as teenagers were recommended age-inappropriate sexual content and self-harm material, claims Meta described as "misleading" and "dangerously speculative".

Meta is also facing a separate trial in New Mexico that began this week.