The case is about whether social media giants are deliberately making their platforms addictive and harmful to children and young people.
"I think it's important to distinguish between clinical dependence and problematic use," Mosseri said in court.
The focus is on a 20-year-old woman whose case could determine how thousands of similar cases may play out in the future.
In the courtroom were parents of children who had taken their own lives after what has been described as addiction to social media.
Instagram owner Meta's CEO, Mark Zuckerberg, is expected to testify in the trial next week.
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