Overall, Facebook's domestic userbase will grow to 173 million people in 2017, a 2.4% increase year-over-year. Now here's the rub: Facebook is seeing "higher usage among older Americans," but lower usage among those aged 12 to 17. That age group, eMarketer predicts, will fall 3.4% to 14.5 million people.
Usage among those under 12 will also grow "more slowly than previously forecast," eMarketer said.
"We see teens and tweens migrating to Snapchat and Instagram," Oscar Orozco, an eMarketer forecasting analyst, said in a statement. "Outside of the Facebook-cutters, teens and tweens remaining on Facebook seem to be less engaged – logging in less frequently and spending less time on the platform."
"At the same time, we now have Facebook-nevers, many children aging into the tween demographic that appear to be overlooking Facebook altogether, yet still engaging with Facebook-owned Instagram," he added.
About 82% of 12- to 17-year-old social users are on Facebook, but come 2021, that number will fall to 76%, eMarketer said. The company also said it is increasing its projections for Snapchat usage. The platform will reach 79 million total monthly users, a 26% uptick from the previous year. Those aged 18-24 will make up 19% of Snapchat's overall growth.
Meanwhile, Facebook-owned Instagram will also see its user thrive: The platform, which has been copying Snapchat features, will see its user base grow 24% this year. Much of the growth will be driven by those under the age of 17, eMarketer said.
Charts by Chen Wu.