The war of Facebook versus Snapchat is as lopsided as it gets. Facebook has proven nearly unstoppable, and lately it's been using a good portion of its might to crush Snap Inc., with dire results for its business performance. But there is one front where the smaller company has been able to win some territory: teens.

Research and analysis firm eMarketer announced on Wednesday that the company "has lowered its projections for Facebook usage for those under 18, while increasing its projections for Snapchat for the same age group." In other words, eMarketer expects more teenagers to turn up their noses at Facebook while eagerly downloading Snapchat.

And that's not the only sign Facebook seems to be losing a touch of momentum. The Verge's Casey Newton notes that downloads of Facebook's iOS app appear to be at their lowest level since 2013, according to data from Apptopia.

But don't start writing those Facebook obituaries just yet. For one thing, along with Snapchat, one of the services that seems to be skimming younger users from Facebook is Instagram, which Facebook owns. Reports eMarketer:

Even as Facebook's US user base continues to grow overall, its share of social network users will continue to decline. It will drop most precipitously for those between 12 and 17 -- 81.9% of US social network users in that age range will use Facebook this year; that figure will drop to 76.2% by 2021.

In contrast, eMarketer has increased its projections for Snapchat usage this year in line with reported data. The platform will grow 25.8% to reach 79.2 million total monthly users in the US. While growth figures were adjusted higher for all but the oldest age group, the biggest adjustment was made to the 18-24 group. That age range will see usage grow 19.2% this year.

Similarly, total Instagram usage will grow 23.8% this year in the US to 85.5 million, more than previously forecast. This year, Instagram will grow its user base among Americans under 12 by 19.0% and those 12-17 by 8.8%.

Even though Instagram has more users overall than Snapchat, the younger social network beats Instagram in two key age groups: 12-17 and 18-24. In fact, eMarketer's latest forecast now -- for the first time -- has Snapchat overtaking both Instagram and Facebook in terms of total users 12-17 and 18-24. As a result, Snapchat's share of social network users will grow to 40.8% this year, reaching nearly half by 2021.

The important thing to bear in mind here is context. Facebook has a problem Snap would love to have. It's pushing against the upper limits of how big a social media network can get.

Back in May, eMarketer estimated that Facebook's revenue acceleration will level off within the next year: "Facebook ad spending is still rising at a fast clip, but the pace of growth will be markedly lower by 2018 than it was in 2016. eMarketer forecasts 26.5% growth in 2018 -- less than half the pace of the 57.4% increase seen in 2016." Slower growth is the natural evolution of a mature company that continues to approach its saturation point in the most profitable markets.

Still dwarfed by the absolute numbers of Facebook, Instagram, and WhatsApp users, Snapchat's one advantage is its cool factor, which leads to its disproportionate popularity among young adults. Teenagers are especially attractive to the brand advertisers that tech companies hope to lure away from TV.

Whereas direct-response marketers seek an immediate purchase or commitment from the consumer, brand advertisers want to drive the amorphous affinity that can lead to a lifetime of consistent purchases. Snapchat is like a miniaturized version of TV, except that it features your friends (and no coherent plots), making it ideal for the brand advertisers moving to digital as TV audiences lag.

It may not be the new-new thing, but Facebook retains every other advantage. Instagram's audacious cloning efforts mean that an ad environment nearly identical to Snapchat's is available on a Facebook property, which means superior targeting and tracking. And contrary to eMarketer's projections, Snapchat's user growth seems to be slowing quarter by quarter:

Tiffany Coletti Kaiser, executive vice president of marketing and client services at consultancy Digital Remedy, assessed the two social media companies earlier in August. "In today's video-focused world, Snap has the right idea that unique experiences are appealing to consumers, and in return to advertisers."

"However, what they get right in terms of ideal offering they fail to understand from an advertiser execution standpoint. Snap hasn't taken into account that the creation of video collateral requires a production budget, time, and assets that most brands don't have," she explained.

In order to be worth breaking past those budget constraints, Snapchat needs to grow. And not just a little bit.