YouTube vice president of engineering Cristos Goodrow was pressed to defend the company's self-styled "big, hairy, audacious goal," set more than a decade ago, to increase viewer time to more than a billion hours a day by 2016.

As he did last week when Meta chief executive Mark Zuckerberg testified in the same Los Angeles court, plaintiff's attorney Mark Lanier told jurors that Goodrow's compensation climbed with his company's share price, meaning he profited personally from ramping up user engagement.

"YouTube is not designed to maximize time," Goodrow replied, as he was shown company documents indicating that viewer engagement was a priority for performance at the platform.

"It's designed to give people the most value..."

As a counterpoint, Lanier had Goodrow detail the addition of features including auto-play for videos and ads, and a version of YouTube designed specifically for children.

The lawyer said these efforts enticed users to a "treadmill of continuous checking" for new content.

The attorney also pointed to internal YouTube documents referencing outside research that found harmful effects from spending too much time watching videos.

The trial is set to last until late March, when the jury will decide whether Meta and YouTube bear responsibility for the mental health problems suffered by Kaley G.M., a 20-year-old California resident who has been a heavy social media user since childhood.

Kaley G.M. started using YouTube at age six, Instagram at nine, and later TikTok and Snapchat.

She is expected to testify this week - perhaps as early as Tuesday, according to her lawyers.

Zuckerberg testified last week that he regretted Meta's slow progress in identifying underage users on Instagram, as the plaintiff's legal team sharply criticized the company for deliberately targeting children.

The trial is the first in a series of lawsuits filed by American families against social media platforms and will determine whether Google and Meta deliberately designed their platforms to encourage compulsive use among young people.

The case is expected to set a standard for resolving thousands of lawsuits that blame social media for fueling an epidemic of depression, anxiety, eating disorders, and suicide.

TikTok and Snapchat, also named in the complaint, reached settlements with the plaintiff before the trial began.