The research, by New York-based firm eMarketer, said that, in 2018, less than half of Americans aged between 12 and 17 will use Facebook at least once a month.
EMarketer said it expects Facebook to lose 2 million users under the age of 25 this year and that Snapchat will pick up 1.9 million users within that age bracket.
Snapchat recently updated its app with a redesign aimed at making the service easier to use. The overhaul kept the social element of the app separate from media, with one page devoted to interacting with friends' Snapchat stories and messages, while another runs a feed of video content from media outlets.
Facebook's monthly user growth is coming from older rather than younger users, the study said.
"Snapchat could eventually experience more growth in older age groups, since it's redesigning its platform to be easier to use," Debra Aho Williamson, principal analyst at eMarketer, said in a statement Monday.
"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 will still be the dominant social network of the two in 2018, however, with 169.5 million U.S. users compared to Snapchat's 86.5 million U.S. users.
Facebook was not immediately available for comment when contacted by CNBC.