Mark Zuckerberg said very little on his first day of testimony—but the fact he’s here at all is a major moment

The social media trial brought by a 20-year-old Californian plaintiff known as Kaley or KGM, putting Meta and YouTube in front of a jury, has captured the world’s attention. The bellwether trial is a test case for the liability of social media platforms and how much they could be on the hook financially if found to have caused harm to their users. KGM, for her part, alleges that she faced anxiety, depression, and body image issues after using Instagram.

The proceedings could establish the first real legal boundaries for what has been up to now largely unregulated algorithmic design, determining whether amplifying harmful content amounts to negligence. A verdict against Meta or YouTube in this bellwether case could open the door to other suits, and finally force disclosure of internal research that has so far remained confidential.

The first day that Mark Zuckerberg, Meta’s CEO, was on the stand on February 18 was a major moment—not necessarily for what Zuckerberg said, but for the fact the case has gotten this far.

“This is a significant moment in terms of these platforms finally being seen to be held to account by their own users,” says Steven Buckley, lecturer in media digital and sociology at City St George’s, University of London.

While Zuckerberg withstood rigorous questioning from Mark Lanier, the lawyer representing Kaley GM, the fact that he was there at all and the case got to trial is a significant happening.

As Fast Company has previously reported, 2026 is the year that the world is getting tough on online safety, particularly for kids. And this trial is notable because it managed to sidestep the usual way social networks swerve liability: Claiming Section 230 protections, which have been in place since the mid-1990s and insulate platforms from bearing responsibility for the actions of their users.

If jurors agree that product design, rather than user behavior, is the root cause of harm, big tech’s decades-long legal shield could begin to fracture. That possibility alone has Silicon Valley watching nervously, with billions in potential damages on the line.

Prior to the trial beginning, Snap and TikTok settled with the claimant without admission of liability, leaving YouTube and Meta to fight the trial.

A Meta spokesperson tells Fast Company the firm “strongly disagree with these allegations and are confident the evidence will show our longstanding commitment to supporting young people,” adding that “the evidence will show she faced many significant, difficult challenges well before she ever used social media.”

YouTube spokesperson José Castaneda tells Fast Company: “The allegations in these complaints are simply not true.”

“It’s not particularly surprising that these large platforms are finally facing some legal repercussions from their actual users,” says Buckley. A steady drumbeat of reporting, alongside other smaller legal cases, have revealed information that suggests social media can be harmful to younger users.

This case is therefore a potential watershed because the plaintiffs argue that Instagram’s and YouTube’s underlying product design—features like the infinite scroll, autoplay and recommendation algorithms that serve up progressively more engaging content—constitutes a defective product.

But most of those other cases haven’t received as much attention because they’ve not gotten as far as this one has—nor have been as likely to succeed in some way.

“Zuckerberg did not come across as someone with children’s best interests at heart,” says Tama Leaver, professor of internet studies at Curtin University in Australia. Leaver contrasts Zuckerberg’s performance in court with Adam Mosseri’s a few days earlier, who the researcher says “had the tenacity to argue that the term addiction is being misused”.

In contrast, “Zuckerberg didn’t feel like someone who’d done their homework, but rather someone who was surprised they had to turn up and answer these questions,” Leaver explains. “If his job was to convince the listening world that he could be a trusted figure in the lives of teens and young people, then he failed.”

Despite that poor performance by Zuckerberg, and despite the strength of the case in comparison to others that have gone before, some think that a decision against the social media firms—or a general movement to recognize the issues inherent with social media—could backfire.

“One concern I have is that people will think that the simple solution to many of the issues raised in these lawsuits is to simply ban under-16s from using the platforms,” says Buckley. “This is a woefully misguided reaction. The scientific evidence regarding the link between social media use at a young age and addiction is still not well established.”

Whether the jury agrees with that assessment or not, the trial has already achieved something that years of congressional hearings and regulatory hand-wringing haven’t: putting the people who designed these systems under oath and making them answer difficult questions—then be responsible for the consequences of what they say.

“One of the reasons I think we have gotten to this stage is that some people have come to the conclusion that their governments are not going to do anything meaningful to hold these companies to account and so have felt compelled to take them on themselves,” says Buckley. The rest of the tech industry will be watching closely to see what comes next.