A bipartisan group of 29 US states is suing Meta, seeking potentially tens or hundreds of billions of dollars in penalties and changes to how Meta does business.
Four lead US states - California, Colorado, Kentucky and New Jersey - accused Meta of designing Facebook and Instagram to hook young users, fuelling anxiety, depression and even suicide, and misleading consumers about the platforms' safety.
All 29 states accused Meta of violating federal law by improperly collecting and using children's personal data.
Experts have called the trial in the Oakland, California, federal court, which began with opening statements on Tuesday, the biggest legal test yet of social media's effects on young users.
Meta and other social media companies including Snap, TikTok parent ByteDance and YouTube parent Alphabet face thousands of lawsuits by states, municipalities, school districts and individuals over whether their products harm young users.
Megan O'Neill, a deputy attorney general for California, told the eight-person jury that Meta's business model was to "hook the users, hold them for as long as they can, harvest their data, and then hide the truth from the public."
"It worked especially well for kids," O'Neill added. "Meta needed kids, and it needed to reassure the people who cared about those kids that the kids are safe."
Meta's lawyer Paul Schmidt said there was "no dispute" some social media users face struggles, but that research showed no clear link between adolescents' social media use and a lack of wellbeing.
He also said Mark Zuckerberg, Meta's co-founder and CEO, shared the company's desire to improve its services, and not make them dangerous.
"They don't believe they're going to do well if people don't like their service," Schmidt said.
Jurors are expected to issue an advisory verdict, but US District Judge Yvonne Gonzalez Rogers will determine Meta's liability.
If she finds Meta liable, Rogers could impose civil penalties and order changes to Facebook and Instagram. Meta has said penalties could be as high as $US1.4 trillion, close to the Menlo Park, California-based company's market value.
Attorneys general said at a hearing last week the amount could be closer to $US200 billion, or about three years of after-tax profit for Meta.
California, Colorado, Kentucky and New Jersey also want Meta to overhaul Facebook and Instagram, including by eliminating likes and the infinite scroll that encourages views of new posts, setting time limits for younger users, and enforcing restrictions to keep children under 13 offline.
After opening arguments concluded, former Meta safety engineer Arturo Bejar began testifying as the states' first witness.
Bejar has long said Meta knew its child safety tools did not work, and has testified against the company in four trials.
Meta sought to block his testimony, allegedly for deleting Signal messages with former employees, but Rogers rejected what she considered a long-shot bid to eliminate a key witness.
Bejar told jurors that "move fast and break things" was a mantra at Meta, which took a "don't ask, don't tell" approach to monitoring whether children under 13 were online.
"Many products were shipped into the world," such as reels short-form videos, "and safety was not a consideration in how it was initially deployed," Bejar said.
Zuckerberg and Instagram chief Adam Mosseri are also expected to testify. The trial is scheduled to last six weeks.
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