A report by eMarketer, a New York-based market research company, 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 percent 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 US would be on Facebook, with a 5.6 percent drop in that segment.
The under-12 age group, meanwhile, will see a decline of 9.3 percent this year.
The same trend is expected to continue into 2019 and 2020.
Facebook will lose an estimated 2 million users under 25 this year, with Snapchat and Instagram the main beneficiaries.
The report said Snapchat will add 1.9 million users under 25 in 2018 and Instagram will add 1.6 million.
Snapchat 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.
"Snapchat could eventually experience more growth in older age groups, since it's redesigning its platform to be easier to use," eMarketer analyst Debra Aho Williamson said.
"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.