It's no surprise that teens hate Facebook, but a new forecast of social network usage in the US and UK by market research firm eMarketer paints a not-so-great picture for the world's biggest social network.

The firm expects Facebook's growth to continue slowing in both countries as "lessening usage among teens and young adults drags down overall user growth." At the same time, eMarketer expects Instagram and Snapchat usage rates to "rise by double digits."

It's not all bad news for Facebook, though. EMarketer says the social network's community of monthly users (aka the number of unique users each month) will grow 2.4 percent this year to 172.9 million people thanks to "increased adoption by older internet users."

But, Facebook's monthly user base between 12- and 17-year-olds—an age group "coveted" by marketers—will fall 3.4 percent this year to 14.5 million, eMarketer predicts. That would represent "the second consecutive year of expected usage declines by this group" following a 1.2 percent drop last year. The firm adds that monthly Facebook usage among those under 12 and ages 18 to 24 will also "grow more slowly than previously forecast."

Meanwhile, Snapchat's US user base is expected to grow 25.8 percent to 79.2 million monthly this year while Instagram's is expected to grow 23.8 percent to 85.5 million. Snapchat's user base of 18- to 24-year-olds will grow 19.2 percent while Instagram is expected to see a 19 percent increase in users younger than 12 and 8.8 percent uptick in users between 12 and 17 years old.

"We see teens and tweens migrating to Snapchat and Instagram," eMarketer Senior Forecasting Analyst Oscar Orozco says. "Both platforms have found success with this demographic since they are more aligned with how they communicate—that is, using visual content.

"Outside of those who have already left, teens and tweens remaining on Facebook seem to be less engaged—logging in less frequently and spending less time on the platform," Orozco adds. "At the same time, we now have 'Facebook-nevers'—children aging into the tween demographic who appear to be overlooking Facebook altogether, yet still engaging with Facebook-owned Instagram."

EMarketer expects Snapchat to overtake both Instagram and Facebook in total users ages 12 to 17 and 18 to 24 for the first time in 2017. "As a result, Snapchat's share of US social network users will grow to 40.8 percent," the firm predicts.

The predictions come after Instagram in April announced that more than 200 million people now use its Stories feature (the one it ripped off from Snapchat) every day. Meanwhile, Snapchat in a February IPO filing revealed that "on average, 158 million people use Snapchat daily."