A landmark social media addiction trial in which top tech executives are expected to testify begins on Tuesday in California.

The plaintiff, a 19-year-old woman identified by the initials KGM, alleges the design of the platforms' algorithms left her addicted to social media and negatively affected her mental health.

The defendants include Meta, which owns Instagram and Facebook, TikTok's owner ByteDance, and YouTube parent Google. Snapchat settled with the plaintiff last week.

The closely watched case at Los Angeles Superior Court is the first in a wave of such lawsuits, which could challenge a legal theory tech firms use to shield themselves from liability in the US.

'Dangerous and addictive algorithms'

The named social media companies have said the plaintiff's evidence falls short of proving they are responsible for alleged harms such as depression and eating disorders.

The case going to trial marks a distinct shift in how the US legal system treats tech firms, which face mounting claims that their products lead to addictive behaviours.

The companies have long argued that Section 230 of the Communications Decency Act, passed by Congress in 1996, exempts platforms from liability for what third parties post.

But the issue in this case is design choices about algorithms, notifications and other features that affect how people use their apps.

KGM's attorney, Matthew Bergman, told the BBC the case will be the first time a social media company has been held to account by a jury at trial.

"Unfortunately, there are all too many kids in the United States, the UK, and around the world who are suffering as KGM does because of the dangerous and addictive algorithms that the social media platforms foist on unsuspecting kids," he said.

"These companies are going to have to explain to a jury why their profits were more important than the lives of our young people."

Eric Goldman, a law professor at Santa Clara University, told the BBC that losing these cases in court could pose an existential threat to the social media companies.

But he said it may be difficult for plaintiffs to prove physical harm can be blamed on content publishers.

"The fact that the plaintiffs have been able to sell that idea has opened the door to a whole bunch of new legal questions that the law wasn't really designed to answer," he said.

'The tech industry has been given deferential treatment'

At trial, jurors are expected to see an array of evidence, including excerpts from internal company documents.

"A lot of what these companies have been trying to shield from the public is likely going to be aired in court," said Mary Graw Leary, a law professor at Catholic University of America.

Meta previously said it introduced dozens of tools to support a safe online environment for teens, but some researchers have disputed the effectiveness of the recent measures.

The companies are expected to argue that any asserted harms are caused by third-party users.

One highly-anticipated witness the jury will hear from is Meta boss Mark Zuckerberg, who is due to testify early in the trial.

In 2024, he told US senators, "the existing body of scientific work has not shown any causal link between social media and young people having worse mental health outcomes".

During that same hearing, at the prodding of one senator, Zuckerberg apologised to victims and their loved ones who had crowded into the chamber.

Tech executives "are often not good under pressure", said Mary Anne Franks, a law professor at George Washington University.

She said the firms were "very much hoping" they could avoid having top bosses testify.

The trial comes as the companies face growing scrutiny from families, school districts, and prosecutors worldwide.

Last year, dozens of US states sued Meta, alleging the company misled the public over risks of social media use and had contributed to a youth mental health crisis.

Australia has enacted a social media ban on under-16s, and the UK signalled in January that it may follow.

"There is a tipping point when it comes to the harms of social media," Franks said.

"The tech industry has been given deferential treatment - I think we're seeing that start to change."