(BSS/AFP) - Jury selection is expected to begin on Wednesday in Los Angeles for another landmark trial about the alleged harms of social media on young users, with Meta and, for the first time, Snap as defendants.

Thousands of lawsuits have been filed in recent years in local, state and federal courtrooms across the country with various claims about alleged harms to children and teenagers caused by social media companies.

The cases -- the first of which went to trial earlier this year in Los Angeles -- have taken twists and turns.

Meta and YouTube lost the first so-called bellwether case earlier this year, when a jury found them collectively liable for harming a young user known by the initials KGM, and ordered the tech companies to pay a combined $6 million penalty.

Meta said at the time that they "respectfully disagree" with the verdict.

Snap and TikTok settled with KGM ahead of that trial.

Months later, Snap, TikTok and YouTube settled another case that was set to go to trial in August; that plaintiff, known as RKC, then withdrew their remaining claims against Meta, allowing the Instagram and Facebook maker to avoid trial without a settlement.

TikTok and YouTube were previously named as defendants in the new case but both companies reached settlement agreements with the plaintiffs, known by their initials SJ and KDB, before the start of the trial, their lawyers said.

Meta reached a settlement agreement with one of the plaintiffs, SJ, but not KDB, according to their lawyers.

A California state judge consolidated both cases into a single trial.

If no additional settlements are reached before opening statements kick off on November 4, the new trial will move forward with both Meta and Snap as defendants.

Star witnesses are expected to testify beginning mid-November including Meta CEO Mark Zuckerberg, Instagram chief Adam Mosseri, and Snap CEO Evan Spiegel.

Earlier this month, a New Mexico judge ordered Meta to pay $25 billion in a case where the company was found liable for deceiving users in the state about how their data was used in a case related to the Cambridge Analytica scandal.

A New Mexico jury previously found the social media giant liable for harms to children, leading to a combined penalty of $942 million.

In August, Meta agreed to pay more than $18 billion in penalties, in addition to making changes to Instagram and Facebook for teens, when it settled with 52 attorneys general several days into a trial held in California.

Part of that settlement hinged on YouTube and TikTok making similar changes to their own products, such as limiting daily screentime.

- `Addictive and problematic` -

As regulators around the world grapple with reining in, or even banning, social media use for teens, American lawsuits have focused on allegations of addictive design and deceptive practices by social media companies.

Next month, lawyers for the two plaintiffs in Los Angeles -- SJ and KDB -- will argue that their clients were exposed to "addictive and problematic" social media apps when they started using them at 13 and 15, respectively, according to court documents.

SJ, a teen in Illinois, began to use Snapchat, the app created by Snap, compulsively and subsequently experienced mental health issues, an eating disorder and self harm, her lawyers claim.

KDB, now an 18-years-old from Mississippi, similarly developed compulsive use of both Meta and Snap, leading to worsening mental and physical health, according to her lawyers.

Meta has previously denied such claims.

"We will not back away from defending ourselves against baseless lawsuits," Meta told AFP in July.

Snap intends to argue that it is different from other apps like Meta`s Instagram and Facebook because of Snapchat`s focus on primarily being a messaging platform.

"We believe the evidence will support that we have put our community first, and that these allegations fundamentally misrepresent the plaintiffs` experiences on Snapchat and how our platform was designed," a Snap spokesperson said in a statement.