The settlements avoid a California trial next month that had been expected to set the tone for hundreds of similar cases.

Meta, which owns Facebook and Instagram, was the last of the four companies to settle, according to court documents filed in federal court in Oakland, near San Francisco.

Snap, TikTok and Google – which owns YouTube – had already settled on May 15, according to documents reviewed by AFP.

The deals come as the legal environment for social media platforms grows increasingly hostile.

In March, a Los Angeles jury ordered Meta and YouTube to pay US$6 million to a young woman, ruling that their platforms were harmfully addictive – a first-of-its-kind verdict.

The day before, a New Mexico jury had ordered Meta to pay US$375 million for exposing minors to inappropriate content and sexual predators.

The Oakland case was brought by the Breathitt County school district, a rural district in eastern Kentucky whose lawsuit had been chosen as a test case for more than 1,200 similar suits filed by school districts across the country.

The district had sought more than US$60 million to offset the costs of dealing with the effects of social media on students – including sleep problems, emotional distress and conflicts – and to fund a 15-year mental health programme.

It had also asked the court to order the platforms to change their algorithms to make them less addictive.

None of the settlements include any admission of wrongdoing.

They are likely to increase pressure on the companies to settle the remaining cases, all of which are being overseen by Judge Yvonne Gonzalez Rogers, who recently presided over the trial between Elon Musk and OpenAI chief Sam Altman.

By settling confidentially rather than going to trial, the four companies also avoided having their internal records aired in open court.

Thousands of social media addiction lawsuits are pending in US courts.

More than 30 states are also suing Meta over similar claims in a separate case that could go to trial in August in Oakland.