Facebook’s user base of 12-17-year-olds in the UK and US will shrink this year, according to a new report.

Young users are “less-engaged” with Facebook than they are with rival social networks, and are therefore leaving the site, logging in less frequently and spending less and less time on it, the report says.

Some children, known as ‘Facebook-nevers’, are even rejecting Facebook altogether, yet still using other social networks.

It’s a hugely concerning outlook for the technology giant, which relies on advertising to generate money.

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According to eMarketer, which describes the age group as “marketer-coveted”, monthly usage figures for 12-17-year-olds will fall by 3.4 per cent to 14.5 million people in the US, and fall by 2.8 per cent in the UK.

It also predicts that monthly usage figures for 18-24-year-olds will fall by 3.1 per cent in the UK this year.

What’s even more worrying for Facebook is that, while eMarketer has reduced its usage estimates for these age groups on Facebook, it says it has either increased or not changed its positive growth estimates for the same age groups on Snapchat and Instagram.

“We see teens and tweens migrating to Snapchat and Instagram,” said eMarketer senior forecasting analyst Oscar Orozco. “Both platforms have found success with this demographic since they are more aligned with how they communicate — that is, using visual content.

“Outside of those who have already left, teens and tweens remaining on Facebook seem to be less engaged — logging in less frequently and spending less time on the platform.”

Fortunately for Facebook, eMarketer predicts that Instagram – which is owned by Facebook – will grow its user base among under-12-year-olds by 19 per cent and 12-17-year-olds by 8.8 per cent in the US.

Snapchat, meanwhile, is expected to overtake both Instagram and Facebook in terms of total users aged 12-17 and 18-24 in the US, for the first time.

However, its outlook for Instagram is more positive than it is for Snapchat, “to reflect Instagram’s seeming success at wooing younger users with Snapchat-like features”.

Instagram ripped Snapchat’s Stories feature last year, and it has proven immensely popular. As of June, Instagram Stories had 250 million daily users, more than Snapchat’s 166 million total number of users.