Facebook-parent Meta will defend itself in a landmark social media trial that begins Tuesday in California, where attorneys have sparred over witnesses and financial penalties.

A coalition of states sued Meta in 2023, and the four states now going to trial were selected to represent them. 

Lawyers for the states will argue Meta deliberately made Facebook and Instagram addictive for children, in violation of state and federal laws. 

In recent days Meta, which has more than three billion users around the world, tried to block ex-Meta employee and expert witness, Arturo Bejar, from testifying. 

Federal Judge Yvonne Gonzalez Rogers dismissed the company's request, handing the states an early victory. In her ruling, she described it as a "Hail Mary" effort to "eliminate a strong witness" for the plaintiffs.

Bejar has previously testified against the company, including in a New Mexico trial which Meta lost.

Lawyers for California, Colorado, Kentucky and New Jersey are expected to question Bejar about Meta's practices around safety and growth, and whether the company publicly misrepresented what it knew, according to a court document.

In a new filing Monday, Meta asked the judge to restrict the scope of testimony from another potential expert witness, Colin Gray, whom the states intend to ask about "dark patterns" -- features designed to manipulate users into making choices preferred by a company. 

Meta founder and chief Mark Zuckerberg is among the star witnesses expected to testify.

Meta "strongly disagrees" with the allegations in the trial, a spokesperson told AFP.

During a hearing last week, a lawyer for the states said they are seeking around $200 billion -- not penalties exceeding $1 trillion, as Meta had claimed in a court filing.

The lawyer said they believe Meta calculated that figure "for shock value."

In addition to financial penalties, the states are demanding changes to Meta's apps. 

Eight people were selected last week to serve on an advisory jury, but the judge will make the final decision in the case. The trial is expected to last six weeks and a verdict is expected by early October. 

Although it's not the first lawsuit that seeks to hold a social media company accountable for mental health and safety issues, it could become one of the most consequential.

Last week, Rogers and lawyers for both sides probed prospective jurors about their views of social media and Meta, asking if they or their children had social media accounts and whether social media is responsible for mental health, among other questions.

One prospective juror said they believed social media plays a role in mental health and compared the serotonin boost that some people feel from "doomscrolling" to using cocaine. 

The case could be "the beginning of a broader reckoning" for Meta, Nora Freeman Engstrom told AFP via email. She is a law professor and associate dean at Stanford. 

It will be important to see the "gap" between what Meta knew privately and what it disclosed publicly, Engstrom said. 

"The huge issue here is reputational harm" and being forced to make major changes, Vincent Joralemon, a director at Berkeley's Life Sciences Law and Policy Center, told AFP.

Experts see parallels with a three-decade-old settlement between dozens of US states and tobacco companies.

"It really feels like tobacco in the 1990s," Joralemon said.

While cases about social media harms revolve around the intersection of technology and addiction, the case against Meta focuses on its business practices, similar to when US regulators sued tobacco companies, Joralemon said.

Dozens of US states sued four major tobacco companies for downplaying the harmful health impacts of their products, and won a 1998 landmark settlement that included financial penalties and changes to product marketing.

Those tobacco companies have paid over $176 billion since, according to data from the National Association of Attorneys General. 

They will continue to pay $9 billion annually, according to the settlement.