A report by eMarketer said Snapchat is drawing youths away from Facebook at a quicker clip than Facebook-owned Instagram.
Facebook is still growing in the US market, according to research firm, mainly due to increases in usage by older age groups.
The report is the latest to highlight Facebook's problem with attracting and keeping young people, who have long been a core user base for the world's biggest social network.
The research firm said it expected the first-ever decline in the 18-24 age group in the US, a drop of 5.8 per cent this year.
It also said that for the first time since its research began, less than half of the 12-17 age group in the United States would be on Facebook, with a 5.6 per cent drop in that segment.
The under-12 age group meanwhile will see a decline of 9.3 per cent this year, eMarketer said.
The same trend is expected to continue into 2019 and 2020, with declines in all segments of US users under 25, the report added.
Snapchat, which is known for its disappearing messages, will continue to have more users aged 12 to 24 than Instagram, the researchers said.
But Snapchat could end up facing a similar problem as it seeks to increase its user base and reach all ages.
"The question will be whether younger users will still find Snapchat cool if more of their parents and grandparents are on it. That's the predicament Facebook is in."
Facebook remains the most popular social network in the US market with an estimated 169.5 million users this year, according to eMarketer.
But faster-growing Instagram will be used by 104.7 million Americans and Snapchat will reach 86.5 million users, according to the forecast.
Last year, eMarketer predicted Facebook would see declines among some youths for the first time in its history.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)