A California jury on Wednesday found Meta and YouTube liable for knowingly using interfaces and features that made the platforms addictive to children and teenagers.
It is a legal decision that could reshape how social media platforms operate and how apps are designed.
Reports indicate that the jury concluded that Meta compensate the plaintiff with about $2 million in damages, and that YouTube would pay $900,000.
The final amount of compensatory damages could be considerably more amid forthcoming hearings related to the case.
For Meta, owner of Facebook, Instagram, WhatsApp and Threads, it is the second major legal loss in two days.
Less than 24 hours beforehand, a judge in New Mexico found the company liable for knowingly having insufficient child protections on the platform, for which Meta now faces a $375 million civil penalty.
The Los Angeles case, however revolved around addiction, and was brought against Meta and YouTube – owned by Alphabet – by a woman who claimed the platforms caused her severe mental harm because of how they were designed.
The plaintiff is now 20 years old and has been referred to by the court as "Kaley".
The two recent legal defeats are rare for Meta, which has mostly avoided any legal repercussions in recent years amid a rising number of accusations and controversy.
"The era of Big Tech invincibility is over – this ruling is an earthquake that shakes Big Tech’s predatory business model to its core," said Sacha Haworth, executive director of Tech Oversight Project.
"These products were purposefully designed to harm, addict millions of young people, and lead to lifelong mental health consequences," she added.
The New York Times quoted a Meta spokeswoman as saying that the company "respectfully disagrees with the verdict" and that the technology giant is currently looking at all of its legal options going forward.
Snapchat and TikTok were initially listed as defendants in the Los Angeles case as well, but ultimately decided to settled out of court.
Common Sense Media, a non-profit which boasts of its goals to "puts kids' safety and well-being first in the digital era", celebrated the verdict, saying that the decision "should embolden lawmakers in California and across the country to use their authority to force real change in how these companies design and operate their products."
