LOS ANGELES — A jury found Meta and YouTube negligent in the design or operation of their social media platforms, producing a bellwether verdict in the first lawsuit to take tech giants to trial for social media addiction.

The jury stated that Meta’s and YouTube’s negligence were a substantial factor in causing harm to the plaintiff, identified in court by her initials, K.G.M., and that the companies failed to adequately warn users of the dangers of Instagram (Meta’s platform) and YouTube (which is owned by Google).

They awarded K.G.M. $3 million in damages, finding Meta 70% responsible for harm caused to the now 20-year-old plaintiff, and YouTube responsible for 30%.

The trial, which began last month in a Los Angeles County courtroom and included testimony from tech executives including Mark Zuckerberg, was the first in a consolidated group of cases brought against that company and others by more than 1,600 plaintiffs, including over 350 families and over 250 school districts.

Outside the courtroom, families who say their children were harmed by social media embraced as they celebrated the verdict, telling reporters that they feel “vindicated.”

K.G.M.’s lead attorney has said he hopes the proceedings produce transparency and accountability “so that the public can see that these companies have been orchestrating an addiction crisis in our country and, actually, the world.”

The plaintiff was a minor at the time of the incidents outlined in her lawsuit. K.G.M. testified in court that her nearly nonstop use of social media caused or contributed to depression, anxiety and body dysmorphia. It “really affected my self-worth,” she said last month.

Speaking about her social media use, K.G.M. testified that she felt she wanted to constantly be on the platforms and feared missing out if she wasn’t.

Attorneys for Meta and YouTube have disputed claims brought forth by the plaintiff, arguing their platforms are not purposefully harmful and addictive.

A spokesperson for Meta said that K.G.M.’s “profound challenges” were not caused by social media and pointed to “significant emotional and physical abuse” that she experienced when she was younger.

In his closing argument, an attorney for YouTube said there was not a single mention of addiction to that platform in K.G.M.’s medical records.

The verdict comes after jurors in a separate trial in New Mexico held Meta liable for failing to protect children from online predators and sexual exploitation on Facebook and Instagram.

The New Mexico jury found on Tuesday that Meta violated the state’s consumer protection laws and ordered the company to pay $375 million in civil penalties. Meta has stated that the company disagrees with the verdict and plans to appeal.

In Los Angeles, deliberations took longer, wrapping up after nearly 44 hours over the course of nine days. The jury had told Judge Carolyn B. Kuhl that they were having trouble coming to a consensus on one defendant.

Social media companies have historically been shielded by Section 230, a provision added to the Communications Act of 1934 that says internet companies aren’t liable for the content users post.

K.G.M.’s lawsuit marked the first civil action seeking to hold the platforms accountable for allegedly causing addiction and mental health problems.

TikTok and Snap, who were also named as defendants in K.G.M.’s lawsuit, reached settlements ahead of the trial. They remain defendants in a series of similar lawsuits expected to go to trial this year.

Matt Bergman, founding attorney of the Social Media Victims Law Center — which is representing hundreds of plaintiffs in state and federal proceedings — told reporters earlier this week that when his firm began suing social media companies four years ago, nobody thought the cases would reach trial.

“But win or lose the outcome of this trial, victims in the United States have won because now we know that social media companies can and will be held accountable before a fair and impartial jury,” Bergman said. “And in some cases plaintiffs will prevail, and in some they may not, but we are just gratified for the opportunity to get this far, and there will be many more trials in the future.”

Shanshan Dong and Angela Yang reported from Los Angeles, and Tim Stelloh from Alameda, California.