Facebook is still adding monthly users but older age groups are mainly responsible for this.
The number of total Facebook users in the US will reach 169.5 million this year, up just under 1 per cent from 2017.
Meanwhile, Facebook's proportion of social network users accessing the platform will continue to decline over the forecast period.
Snapchat is set to gain from Facebook's loss.
"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.
In 2018, the number of US Facebook users ages 11 and younger will decline by 9.3 per cent.
Additionally, the number of users ages 12 to 17 and 18 to 24 will decrease by 5.6 per cent and 5.8 per cent, respectively.
"Facebook will lose two million users ages 24 and younger this year", it said.
But not all of those users are migrating to Instagram.
Instagram will add 1.6 million users ages 24 and younger.
"Snapchat, meanwhile, will add 1.9 million users in that age group. Snapchat will continue to have more users ages 12 to 24 compared with that of Instagram," the report added.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)