On Monday, Instagram's owner Meta and Google-owned YouTube went on trial in Los Angeles County Superior Court in one of the first US jury cases accusing the platforms of deliberately designing addictive products that damage young users' mental health.
TikTok and Snap, which were initially named in the case, settled earlier for undisclosed sums.
The case is being treated as a bellwether that could shape hundreds of similar lawsuits nationwide.
What are Meta, YouTube accused of?
Mark Lanier, the lawyer representing 20-year-old plaintiff identified by her initials "KGM" told jurors the companies "engineered addiction in children's brains," citing internal documents and research.
The plaintiff KGM contends that she suffered severe mental harm after she became addicted to social media as a child.
According to Lanier, she began using YouTube at six and Instagram at nine, posting 284 YouTube videos before finishing elementary school.
"They don't only build apps; they build traps," Lanier said.
How did Meta, YouTube defend themselves?
Lawyers for Meta countered that there is no scientific consensus on social media addiction and argued the plaintiff's mental health struggles stemmed from family issues, emotional abuse, body image issues and bullying, and not platform use.
Jose Castaneda, a YouTube spokesperson, said "the allegations in these complaints are simply not true."
Executives including Meta head Mark Zuckerberg and Instagram boss Adam Mosseri are expected to testify during the trial which is expected to last several weeks.
Tech companies deny the allegations, citing safeguards for young users. They are also arguing that they are protected from liability under US law.
The verdict could have far-reaching implications for how social media platforms are regulated and designed for children.
Debates rage over restrictions on social media for children
Meanwhile, opening statements began on Monday in a New Mexico trial alleging that Meta failed to protect young users from sexual exploitation.
In June, a bellwether trial will begin in Oakland, California, where school districts are suing social media platforms over the harm they have caused children.
These US lawsuits come at a time when countries around the world are debating restrictions on social media for children.
Last year, Australia banned the use of social media platforms by kids under 16. France's ban for children under 15 is coming into force in September.
Meanwhile, preliminary findings published by the European Commission on Friday called TikTok's "addictive design" a breach of European Union law.
