LOS ANGELES — In a highly-anticipated testimony Thursday, the plaintiff in a landmark social media addiction trial said using apps like Instagram and YouTube when she was a child fueled her depression and anxiety, leading her to withdraw from her family.

Now 20 years old, the plaintiff — identified in court documents by her initials, K.G.M. — detailed how her almost nonstop use of social media “really affected my self-worth.”

“They made me give up a lot of hobbies and old interests, and they prevented me from making friends … [and] caused me to compare myself to other people,” she told jurors in Los Angeles County Superior Court.

“I just felt like I wanted to be on it all the time,” she said. “If I wasn’t on it, I was going to miss out on something.”

K.G.M.’s trial is the first in a consolidated group of cases brought against Instagram, YouTube, TikTok and Snap by more than 1,600 plaintiffs, including over 350 families and over 250 school districts. The plaintiffs accuse the tech companies of knowingly designing addictive products harmful to young users’ mental health.

Historically, social media platforms have largely 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. TikTok and Snap reached settlements with K.G.M. before the trial, but they remain defendants in a series of similar lawsuits expected to go to trial this year.

K.G.M.’s bellwether case could set a legal precedent for whether social media platforms are liable for mental health issues in children. If the jury’s verdict favors K.G.M., the companies could face damages to be determined by the jury and forced to change the designs of their platforms. The verdict could also set the tone for whether they choose to fight or settle the oncoming cases.

Meta, which owns Instagram and Facebook, and Google, which owns YouTube, have denied that their apps are purposefully harmful and addictive for young users.

In court, K.G.M. elaborated on the claims that the companies made deliberate design choices to make their platforms more addictive to children for purposes of profit. Her complaint highlights a variety of features that it argues the platforms use to “exploit children and adolescents,” including “an algorithmically-generated, endless feed to keep users scrolling,” rewards that encourage people to keep using the platform and “incessant” notifications, as well as “inadequate” measures for age verification and parental control.

K.G.M. said she created her Instagram account at age 9, before the app asked new users to enter their birthdays. Before that, she said, she also lied about her age to create a YouTube account without having read through any of the platform’s legal fine print.

As she was growing up, K.G.M. told jurors, being without her phone often sent her “into a panic.”

“Without it, I felt like a huge part of me was missing,” she said. “If I didn’t have it, I would be missing out on something. I couldn’t see who was liking my stuff.”

Though she and her mother often argued about her phone use, she couldn’t put it down. She said she would get “a rush” every time she got a notification about her social media posts or profiles and subsequently felt so compelled to check them that she’d sneak away to the bathroom or put off sleep at night.

“When I got a bunch of likes, I was really happy,” she said. “If I didn’t get a lot of likes, I would feel I shouldn’t have posted it, I was ugly.”

With YouTube, K.G.M. said, she started “at a young age” and “spent all my time on it.”

“I would watch it in class,” she said. “Any time I tried to set limits for myself, it just didn’t work, and I just couldn’t get off.”

K.G.M. said that because of her social media use, she still struggles with body dysmorphia. She said she began experiencing it after she was exposed to social media filters, which often overlay effects like skin smoothing or makeup.

Meta introduced beauty filters to Instagram stories in 2017. In 2019, it significantly expanded its slate of augmented reality filters, allowing users to make and publish their own.

K.G.M. said she now tries to avoid filters “because I know I’ll feel worse if I use them.”

Meta has pushed back against claims that the design of social media platforms is responsible for K.G.M.’s mental health challenges as a child, arguing in a brief filed Wednesday that she faced other issues at home that contributed to her mental state.

Its filing pointed to “numerous examples of ‘emotional abuse and neglect by [Plaintiff’s] mother, including prolonged periods of the silent treatment, frequent name-calling (e.g., ‘dumb,’ ‘stupid’), and mocking of her voice,’ and ‘physical abuse, including hitting the plaintiff.’”

(In court, K.G.M. disputed Meta’s argument that her family contributed to her mental health struggles, telling jurors she didn’t experience “abuse or neglect or anything like that.”)

Pressed about social media addiction, Instagram head Adam Mosseri, who testified earlier this month, said, “I think it’s important to differentiate between clinical addiction and problematic use.”

Mosseri also touched on filters, saying the platform eventually decided to prohibit “effects promoting plastic surgery.”

YouTube’s vice president of engineering, Cristos Goodrow, who took the stand Monday and Tuesday, also emphasized that the video platform is “not designed to maximize time.”

On Wednesday, Victoria Burke, who was K.G.M.’s therapist when she was 13, testified that she doesn’t think social media was the sole driver of her former client’s mental health issues but that she does suspect it played a part.

“I believe it was a contributing factor, not a causation factor,” she said.