James Martin/CNET

"Facebook is for old people" has long been a mantra in my teenage-filled household. But now eMarketer has proved how that statement is evidence of an ongoing trend.

The research firm released a forecast Monday saying that Facebook is losing younger users at an even faster pace than previously expected. And even though the social network has been able to retain the teenage audience through its Instagram platform, Snapchat appears to be siphoning them away.

In 2018, for the first time, eMarketer is predicting that less than half of US internet users ages 12 to 17 will use Facebook at least once per month.

The social network will lose 2 million users ages 24 and younger, the research firm predicts. Meanwhile, Instagram will add 1.6 million users and Snapchat will add 1.9 million users in this age group.

Facebook continues to add monthly users overall. It reigns supreme among social networks in the US and is expected to reach 169.5 million users this year. But its new users tend to be in older age groups, eMarketer said.

Instagram is still bigger in the US than Snapchat. It's user total is expected to grow by 13.1 percent to reach 104.7 million this year. Snapchat is expected to grow by 9.3 percent to 86.5 million users.

But Snapchat, which recently updated its app with a redesign aimed at making the service easier to use, could gain more older users.

"The question will be whether younger users will still find Snapchat cool if more of their parents and grandparents are on it," said eMarketer principal analyst Debra Aho Williamson. "That's the predicament Facebook is in."

Facebook declined to comment on the eMarketer report.

A Piper Jaffray report released in April also showed Snapchat's growing popularity among teens. It said that Snapchat edged out Instagram in both monthly usage and as the favorite social platform among teens in the US. Snapchat actually saw both numbers rise, while Instagram held steady with usage and fell as a favorite.

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