Meta Platforms, the parent company of Instagram and Facebook, has gone on trial in one of the most consequential legal challenges yet to the way social media platforms treat children.

Attorneys representing US states alleged in a California federal court on Tuesday that Meta exploited the vulnerabilities of young users, designed features to maximise engagement and failed to tell the public the full truth about risks to children.

Meta has rejected the allegations, arguing that it has introduced extensive safeguards for teenagers and that the states have failed to establish the harms they claim.

The case could have implications far beyond a financial penalty. The states are seeking changes to features and practices at the heart of how Facebook and Instagram operate.

What is Meta accused of?

The case brings together claims involving 29 US states.

At the heart of the lawsuit are three major allegations:

• Meta designed Facebook and Instagram in ways that encourage compulsive use among children and teenagers.
• The company allegedly misled consumers about the safety of its platforms for young people.
• Meta allegedly collected and used personal information belonging to children under 13 without parental consent, in violation of US federal children's privacy law.

California Deputy Attorney General Megan O'Neill told the court that Meta had exploited children's developmental vulnerabilities in pursuit of growth and profit.

The allegations have not yet been decided in this case.

Why are Instagram's design features under scrutiny?

The trial is not simply about harmful posts appearing on social media.

A much bigger question is whether the design of the platform itself can cause harm.

Features such as infinite scrolling, notifications, recommendation systems and engagement mechanisms have come under scrutiny because they can encourage users to remain on an app for longer periods.

The states allege Meta understood the particular vulnerabilities of younger users but continued using engagement-focused features.

Meta disputes the characterisation of its products as deliberately addictive.

Children's data is another major part of the case

The trial also involves allegations under the US Children's Online Privacy Protection Act, commonly known as COPPA.

The states accuse Meta of improperly collecting personal data from users under 13 without the parental consent required by federal law.

This makes the trial about two connected but distinct questions:

Did Meta's products harm young users?

And:

Did Meta unlawfully collect children's information?

What does Meta say?

Meta denies wrongdoing.

The company says it has spent years developing tools and policies intended to make Facebook and Instagram safer for teenagers.

Meta argues that the states have not adequately proved that its platforms caused the alleged harm and says the lawsuit overlooks the safety measures it has introduced.

The company has also challenged the scale of the penalties being sought.

Zuckerberg and Instagram chief may testify

Some of the biggest names in the technology industry are expected to feature during the trial.

Meta CEO Mark Zuckerberg and Instagram head Adam Mosseri are expected to testify.

The proceedings are expected to last around six weeks.

US District Judge Yvonne Gonzalez Rogers is presiding over the case.

An unusual feature of the proceedings is that the jury is advisory. Its findings can guide the judge, but the ultimate determination of Meta's liability rests with Judge Rogers.

Could Meta really face a $1.4 trillion penalty?

The potential financial exposure has attracted enormous attention.

Meta has previously said that the states' approach to calculating penalties could produce a figure of as much as $1.4 trillion.

That does not mean Meta has been fined $1.4 trillion or that such an award is inevitable.

The eventual amount, if Meta is found liable, could be substantially different. The states are also seeking changes to Meta's products and practices, making the case potentially more significant than the headline damages figure alone.

This is not Meta's first child-safety courtroom battle

The Oakland trial comes during a difficult year for Meta in US courts.

Earlier this month, a New Mexico court ordered Meta to pay $567 million into a fund aimed at teen mental-health prevention and treatment, following an earlier $375 million jury award on related consumer-protection claims.

Meta has said it intends to appeal the New Mexico ruling.

In another case earlier this year, a Los Angeles jury found Meta and Google liable in a social-media addiction lawsuit.

The latest trial is therefore part of a much wider legal challenge confronting social-media companies over their relationship with children.

Why could this trial matter outside America?

The central question raised by the case is increasingly relevant worldwide:

How much responsibility should a social-media company bear for the way children use its product?

Until now, much of the debate over children's social-media use has focused on parents, screen-time limits and harmful online content.

Cases such as this one shift attention towards the companies themselves and the architecture of their products.

If the states succeed in forcing significant changes to Meta's platforms, the consequences could extend beyond American teenagers because Facebook and Instagram are global products.

The trial could therefore influence the wider debate over age verification, notifications, recommendation algorithms, children's data and limits on engagement-focused design.

What happens next?

The trial is expected to run for several weeks, with testimony from Meta executives, state witnesses and experts.

The states will attempt to prove that Meta knew about risks to young users, failed to adequately disclose them and violated laws protecting children.

Meta will try to demonstrate that its products were not unlawfully designed to harm children and that it has taken substantial steps to protect teenagers.

The eventual ruling could determine not just how much Meta might have to pay, but whether courts can force fundamental changes in the way some of the world's largest social-media platforms are designed for children.