Oliver Coghlan embodies Facebook’s problems with teen and young adult audiences – a growing number of them do not like it. The 23-year-old says he stopped using Facebook regularly three years ago and he is considering deleting the app. His sole use for it now is to check people’s birthdays.

“I haven’t deleted it yet but I might do soon – I really don’t like the company’s monopolistic behaviour,” said Coghlan, a British student based in the Netherlands. He added that the EU referendum and the 2016 US presidential election, and the online anger that accompanied those polls, convinced him that he wanted to spend less time on Facebook’s main platform.

“There were comments that would come up from people arguing about stuff they don’t know about.”

Facebook’s problem is that Coghlan, who says he would also leave the company’s Instagram app if he could find an alternative, is not the only one. According to internal documents leaked by whistleblower France Haugen, who delivered hard-hitting testimony to US senators on Tuesday, Facebook is struggling to recruit and retain a young audience.

“Facebook understands that if they want the company to grow they have to find new users,” she told senators.

The company’s own research, leaked by Haugen, shows that Facebook is having demographic problems. A section of a complaint filed by Haugen’s lawyers with the US financial watchdog refers to young users in “more developed economies” using Facebook less. It quotes an internal document stating that Facebook’s daily teenage and young adult (18-24) users have “been in decline since 2012-13” and “only users 25 and above are increasing their use of Facebook”.

Further research reveals “engagement is declining for teens in most western, and several non-western, countries”. Haugen said that engagement is a key metric for Facebook, because it means users spend longer on the platform, which in turn appeals to advertisers who account for $84bn (£62bn) of the company’s $86bn in annual revenue.

In her testimony, Haugen described how Instagram was key for recruiting younger users and that she “would not be surprised” if recently paused plans to build an app for 10- to 12-year-olds, Instagram Kids, were revived. The plans were shelved after the Wall Street Journal, which was given access to internal documents by Haugen, revealed that Facebook knew from its own detailed research that Instagram was having a negative impact on the mental health of some teenage girls.

Young demographics are vital for social media companies, according to Ygal Arounian at Wedbush Securities, a US financial firm, because they want loyal users to grow older with their platforms, which appeals to advertisers looking to shape buying decisions. In that context, attracting users on to Messenger Kids (for six to 12-year-olds) or Instagram (for those aged 13 and older) or Instagram Kids if it ever launches, is commercially advantageous if over time they gravitate on to Facebook and its 1.9 billion daily users worldwide. Instagram has more than 1 billion users.

“Younger demos are critical to most platforms, not just Facebook,” says Arounian. “The below 13 demo [demographic], that was the cause of much consternation with the Instagram for kids app, isn’t necessarily key, but platforms in general want to capture younger audiences, and create a loyal audience as they grow older. Advertisers as well often value younger demos more.”

If Coghlan’s decision to use Facebook less is a reflection of wider young audience behaviour, and according to the company’s documents it is, then it could signal trouble for the company. One of the leaked documents states that during Covid, “every cohort’s use of Facebook increased, except for those 23 and under, which continued to decline.”

In the UK, the government is putting forward the online safety bill, which imposes a duty of care on social media companies to protect users from harmful content. It means, at least in the UK, that there will be greater scrutiny of any attempt by Facebook to launch youth-oriented products – whether via its eponymous platform, Instagram, the WhatsApp messaging app or its Oculus virtual reality service.

Beeban Kidron, the crossbench peer who sits on the joint committee looking into the online safety bill and was behind the recent introduction of a children’s online privacy code in the UK, says Haugen’s testimony has “galvanised” public opinion.

“The UK is already leading the world in developing regulatory approaches for the digital age and what we have seen this week really helps galvanise public opinion and it helps us work with lawmakers in other parts of the world, particularly in the US, to get an accountable digital world. This is not about saying we don’t want digital engagement for children. We want digital engagement for children but we just want it on a basis that respects rights and safety.”

Facebook said its most recent figures, which do not split out users by age, showed that its worldwide daily audience continued to grow, up 7% in the three months to 30 June, with the combined daily audience (including Instagram and WhatsApp) growing 12% to nearly 2.8 billion. It also referred the Guardian to a blogpost in which the company said businesses that operate in a competitive field do target young audiences.

“Companies that operate in a highly competitive space … make efforts to appeal to younger generations. Considering that our competitors are doing the same thing, it would actually be newsworthy if Facebook didn’t do this work.”

• This article was amended on 9 October 2021 to correct the spelling of Oliver Coghlan’s surname.