Meta, which owns Instagram, and Google, which owns YouTube, are accused of deliberately making their platforms addictive and harmful to children and young people.
"I will prove that these companies built machines designed to make children's brains addicted and that they did so on purpose," said prosecutor Mark Lanier.
Initially, the allegations were also directed at TikTok and Snapchat, but both settled.
The trial is taking place in California, and the outcome could ultimately affect the question of what responsibility the companies bear for material shared on their platforms.
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