Perhaps more importantly, they also indicate that Facebook may have a tough time attracting a new generation of social media users. According to commentators, it is a grown-up problem for the social network, which needs young users to develop the habit of checking Facebook to show them advertising into adulthood.
The research firm said it expected the first-ever decline in the 18-24 age group in the United States, a drop of 5.8 per cent this year.
eMarketer principal analyst Debra Aho Williamson said: "Snapchat could eventually experience more growth in older age groups, since it's redesigning its platform to be easier to use". In 2016, Facebook's user base for 18- to 24-year-olds fell by 1.5 percent.
And not all users are expected to go towards Facebook-owned Instagram, which will add 1.6 million users under the age of 25 for 2018, according to eMarketer.
Snapchat, in a bid to arrest its declining user growth after the viral success of Instagram Stories, has refreshed its app to make it more user-friendly. Snapchat is expected to grow by approximately 9 percent to 86.5 million users.
A recent report by eMarketer, a New York-based market research company specializing in digital marketing, media and commerce, revealed that younger users are being pulled from Facebook to Snapchat at an even faster pace than they're being siphoned off by Instagram.
"This is a logical effect of the "ageing" of Facebook as a proposition and a well-known environment, and the inevitable emergence of newer social platforms offering the buzz of new features and functions", Karina von Abrams added, principal analysist at eMarketer.More news: South African police raid home of Gupta family amid corruption probe
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In fact, Snapchat could even see increased interest from older groups now.
While Facebook is still adding users overall, most of these are in older age groups. That represents a decline of almost 10 percent, or roughly three times what analysts had predicted.
"Snapchat has also introduced a range of interactive features that have appealed to teen audiences, in turn driving up the user number".
A surge in older users means over-55s will become the second-biggest demographic of Facebook users this year. This compared to 24 percent for Instagram, 9 percent for Facebook and 7 percent for Twitter Inc (NYSE: TWTR).
Looking ahead, Marketer believes the decline will not be quite as high in 2018. but still concerning for Facebook at an estimated 2.1 million users in the same demographics. That likely has executives anxious about the long-term dominance of the social media platform.