On a balmy morning in Los Angeles, a group of 13 parents gathered last week on the steps of a state courthouse, silently holding photographs of their deceased children. They lost them, the parents argued, due to the same defective, destructive force: social media.

Each family has filed lawsuits alleging that the apps their kids used — Instagram, TikTok, YouTube, Snap — were addictive not by chance but by design. That they are dangerous products, like cigarettes or cars without seatbelts, and the firms that made them must be held liable for the destruction wrought. The social media companies strongly reject the allegations.

The first of thousands of these cases, brought by a 19-year-old woman known by her initials KGM, will kick off this week with opening statements in the Los Angeles Superior Court once jury selection is completed. She claims that her compulsive use of Instagram and YouTube, starting at six years old, led to depression, body dysmorphia and risk of suicide. Meta Platforms chief Mark Zuckerberg is expected to testify.

“This is our Big Tobacco moment,” said Joann Bogard, who flew to the West Coast from Indiana in the midwest of America to stand in solidarity with other grieving parents on the courthouse steps. Her son, Mason, died seven years ago when he attempted a viral “choking” challenge. He was 15 years old.

“Parents across the country are going to actually see the evidence that these products are harmful,” she said. “These companies perfectly design their algorithms to create that addictive piece. Just like Big Tobacco, you create addiction and keep those consumers for life.

“Unfortunately, many of those consumers don’t get to have a long life. They die from this product. That’s how my son died. The algorithm chose to send him harmful content, unsolicited, and this is why we don’t have him here today.”

Advocates see the case as a potential turning point after years of fruitless attempts to rein in the social media giants. Filed four years ago by KGM, the case was chosen by the Los Angeles judge as a bellwether of the more than 1,200 lawsuits that have been filed across state courts in the US.

For the social media giants, the stakes could scarcely be higher. Because the evidence and many of the arguments in this trial will be broadly similar to those in the suits primed to follow, the verdict will be seen as a sign of what is to come. A victory for KGM could open the floodgates, while a defeat could dash the hopes of plaintiffs still in the queue.

“This is a huge deal,” said Julia Duncan of the American Association for Justice, a legal advocacy group. “If these cases are successful, the platforms will have to reassess how they design their products, and how they market them, to avoid facing similar lawsuits.”

It could also lead, advocates said, to a master settlement akin to the one from the tobacco companies when they agreed, in 1998, to pay $206 billion into a fund to cover the health and societal costs of smoking.

The case will be the first to make it to trial that is centred on product liability (how apps are designed and built) as opposed to the content they show. It is an important distinction, because social media companies have in previous cases successfully leaned on the broad immunity granted to them under Section 230 of America’s Communications Decency Act, a 30-year-old law that shields companies from liability that may arise from what third parties post on their websites.

• Social media killed my sister — here’s what needs to change

TikTok and Snap, named as defendants in the KGM case alongside Meta and YouTube, have separately struck out-of-court settlements in the past fortnight. At least eight other bellwether cases will be heard in the same southern California court after this one.

The trial represents just the beginning of the social media giants’ legal fight. A separate set of federal cases, brought by school districts and state attorney generals, will go to trial this summer in a district court in Oakland. That parallel process represents the consolidation of a further 1,200 cases filed in federal courts.

The courtroom reckoning coincides with a broader societal shift. In December, Australia became the first country to ban the use of social media platforms by under-16s. And in the UK the House of Lords last week backed an amendment to the Children’s Wellbeing and Schools Bill that would institute a similar smartphone prohibition in schools. Tory shadow education minister Baroness (Diana) Barran called smartphones “the gateway drug to social media”.

The European Commission on Friday threatened to fine Tiktok after its investigators issued a preliminary finding that the app’s “addictive design” breached child protection rules. Tiktok called the allegations “meritless” and vowed to challenge the investigation.

“This is a big moment, no matter what happens,” said Laura Marquez-Garrett of the Social Media Victims Law Center in Washington. “What the world’s about to see over the next several weeks and months are documents, expert testimony, former employees, current employees, people having to account for the things they said and did.

“And that’s what we’ve never had when it comes to these products. We’ve never had transparency.”

The trial will also mark the biggest test yet of the argument long made by the social media industry — that correlation is not causation. Suicide rates among 10 to 24-year-olds surged by 62 per cent in America between 2007, the year the iPhone came out, and 2021. Rates of depression, anxiety and other mental health challenges also increased steeply over the period, which overlapped with the rise of social media.

Executives, however, have long argued that there is no direct evidence that social media was to blame. Indeed, when Zuckerberg testified to Congress in 2024, he said: “There’s a difference between correlation and causation.”

Jonathan Haidt, a psychologist and author of The Anxious Generation, a book that claims social media is bad for kids, said the line of argument pursued by Zuckerberg no longer holds water. In a recent post he collated 31 internal Meta reports, either leaked by whistleblowers or turned over in legal cases, into a single place, MetasInternalResearch.org.

Haidt also authored a separate report that pulled together a “flood” of new research purporting to show direct links between social media use and a catalogue of harms to young people. “Social media companies are harming young people on an industrial scale,” he said, adding that it was “not safe for children and adolescents. The evidence is abundant, varied and damning.”

Meta said: “We strongly disagree with these allegations and are confident the evidence will show our longstanding commitment to supporting young people. For over a decade we’ve listened to parents, worked with experts and law enforcement, and conducted in-depth research to understand the issues that matter most. We’re proud of the progress we’ve made, and we’re always working to do better.”