According to the allegations, Meta and Alphabet have built services aimed at getting children to continue using them, with personalized algorithms and an almost perpetual flow of content that demands attention.
A young woman claims that as a child she became addicted to apps such as Instagram and YouTube.
The verdict states the woman should be awarded $3 million (28 million SEK) in damages. Meta will be responsible for 70 percent of the amount and Alphabet's YouTube for the rest.
The next phase of the trial, which is expected to begin shortly, will determine whether additional damages should be awarded.
The jury found that the companies had not adequately warned about the dangers of using the platforms.
Zuckerberg admitted shortcomings
When he testified in February, Meta CEO Mark Zuckerberg admitted that his company should have been faster in identifying underage users on Instagram and in adjusting the service accordingly.
The trial began at the end of January in Los Angeles Superior Court.
It is unclear whether the ruling will be appealed. A Meta spokesperson said in a statement: "We respectfully disagree with the ruling and are evaluating our legal options."
The case could have repercussions for social media and the broader technology market.
Meta also found liable in New Mexico
It is one of several similar lawsuits this year. The trials have been called the social media industry's "big tobacco" moment, comparing them to the 1990s when tobacco companies were forced to pay billions of dollars for lying to the public about the potential harms of their products.
On Tuesday, a jury in New Mexico awarded Meta the equivalent of just over SEK 3.5 billion in damages. The company is believed to have concealed what it knew about the dangers of sexual exploitation of children on its platforms.
That ruling will be appealed by Meta.
