The number of British social media users aged between 12 and 24 who are regularly logging in to Facebook will fall by 700,000 to 6.7 million this year, leaving the company with just 71% of the market share, according to EMarkerter research.
Users are increasingly moving to Snapchat, where it is predicted that 43% of British teenagers will log in over the course of 2018 - more than double the numbers in 2015.
The statistics are equally true in the US, where for the first time ever fewer than 50% of teenage internet users will log in to Facebook at least once a month.
Facebook is still adding monthly users, but they are mainly from older age groups - and not all the young people leaving the platform are going to Facebook-owned Instagram, which is not growing as fast as Snapchat.
Facebook's slump follows a move by the company to prioritise "more meaningful social interactions", according to chief executive and founder Mark Zuckerberg.
Shares fell 4% after he announced the shift.
Mr Zuckerberg said "video and other public content have exploded on Facebook in the past couple of years", meaning there is "more public content than posts from your friends and family".
That, he said, has shifted the balance "away from the most important thing Facebook can do - help us connect with each other".
This time last year, the Children's Commissioner warned that the "incomprehensible" terms and conditions of social networks meant that children had little idea of what they were signing up to.