The verdict could mark a turning point in the global backlash against their platforms' perceived mental health harms to youth, more than two decades after the emergence of social media.
The jury found Meta liable for $US4.2 million in damages and Google for $US1.8 million, small amounts for two of the world's most valuable companies.
Meta will be liable for 70 per cent of the damages and Google for 30 per cent.
The case involves a 20-year-old woman who said she became addicted to Google's YouTube and Meta's Instagram at a young age because of their attention-grabbing design.
The jury concluded Google and Meta were negligent in the design of both apps and failed to warn about their dangers.
"Today's verdict is a referendum - from a jury, to an entire industry - that accountability has arrived," the plaintiff's lead counsel said in a statement.
Shares of Meta closed up 0.3 per cent and Alphabet shares finished 0.2 per cent higher, following the verdict and damages awards.
Meta disagrees with the verdict and its lawyers are "evaluating our legal options," a company spokesperson said.
Google plans to appeal, company spokesman José Castañeda said.
He said the verdict misrepresents YouTube "which is a responsibly built streaming platform, not a social media site."
The plaintiffs in the Los Angeles proceeding focused on platform design rather than content, making it harder for the companies to avert liability.
Snap and TikTok were also defendants in the trial.
Both settled with the plaintiff before it began.
Terms of the agreements were not disclosed.
Large technology companies in the United States have faced mounting criticism in the last decade over child and teen safety.
The debate has now shifted to courts and state governments.
The US Congress has declined to pass comprehensive legislation regulating social media.
At least 20 US states enacted laws last year on social media usage and children, according to the nonpartisan National Conference of State Legislatures, an organisation that tracks state laws.
The legislation includes bills that regulate the use of mobile phones in schools and require users to verify their ages to open a social media account.
NetChoice, a trade association backed by tech companies such as Meta and Google, is seeking to invalidate age verification requirements in court.
A separate social media addiction case brought by several US states and school districts against technology companies is expected to go to trial this year in federal court in Oakland, California.
Another state trial is slated to begin in Los Angeles in July, said Matthew Bergman, one of the lawyers leading the cases for the plaintiffs.
It will involve Instagram, YouTube, TikTok and Snapchat.
Separately, a New Mexico jury on Tuesday found Meta violated state law in a lawsuit brought by the state's attorney general, who accused the company of misleading users about the safety of Facebook, Instagram and WhatsApp and of enabling child sexual exploitation on those platforms.
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