The question of addiction is a key pillar of the case, where plaintiffs seek to hold social media companies responsible for harms to children who use their platforms.
Meta Platforms and Google's YouTube are the two remaining defendants in the case, which TikTok and Snap have settled.
At the core of the Los Angeles case is a 20-year-old identified only by the initials "KGM", whose lawsuit could determine how thousands of similar lawsuits against social media companies would play out.
She and two other plaintiffs have been selected for bellwether trials - essentially test cases for both sides to see how their arguments play out before a jury.
Adam Mosseri, who has headed Instagram since 2018, said it was important to differentiate between clinical addiction and what he called problematic use.
The plaintiff's lawyer, however, presented quotes directly from Mosseri in a podcast interview a few years ago where he used the term addiction in relation to social media use, but he clarified he was probably using the term "too casually", as people tended to do.
Mosseri said he was not claiming to be a medical expert when questioned about his qualifications to comment on the legitimacy of social media addiction, but said someone "very close" to him had experienced serious clinical addiction, which was why he said he was "being careful with my words".
He said he and his colleagues use the term "problematic use" to refer to "someone spending more time on Instagram than they feel good about, and that definitely happens".
It's "not good for the company, over the long run, to make decisions that profit for us but are poor for people's wellbeing", Mosseri said on Wednesday.
During cross examination, Mosseri and Meta lawyer Phyllis Jones tried to reframe the idea that Lanier was suggesting in his questioning that the company is looking to profit from teens specifically.
Mosseri said Instagram makes "less money from teens than from any other demographic on the app", noting that teens did not tend to click on ads and many did not have disposable income that they spent on products from ads they received.
Questioning Mosseri for a second time, Lanier was quick to point to research that showed people who joined social media platforms at a young age were more likely to stay on the platforms longer, which he said made teen users prime for meaningful long-term profit.
"Often people try to frame things as you either prioritise safety or you prioritise revenue," Mosseri said.
"It's really hard to imagine any instance where prioritising safety isn't good for revenue."
Meta CEO Mark Zuckerberg is expected to take the stand next week.
Meta is also facing a separate trial in New Mexico that began this week.
