So maybe Facebook isn’t perfect after all.

Mark Zuckerberg’s social-media powerhouse has seemed impregnable as of late, cruising through its second quarter results with all cylinders still firing and an unmatched audience over 2 billion worldwide. But then along comes eMarketer with a damning estimate, forecasting unprecedented declines among teens and tweens in the U.S. and U.K. next year. What’s worse, rival Snapchat is projected to be growing in the same demographics.

But those who have paid attention to Facebook going back to its first days on Wall Street know this is not a new criticism. There’s always been something headline-friendly about this narrative, that the mighty Facebook still has an Achilles heel–ironically a vulnerability rooted in the very same young people who brought Zuckerberg’s extra-curricular activity at Harvard to global prominence in the first place.

But is Zuckerberg sweating eMarketer? Surely this is something that Facebook management has been aware of far before eMarketer caught on, an audience trend borne out of the sophisticated data that describes who is active on the platform.

This just puts a new wrinkle on an old concern, one that in part prompted Facebook’s acquisition of Instagram way back in 2012 (not coincidentally, eMarketer sees teen/tween growth there, too). It’s why more recently you are seeing Instagram and Facebook copy the features that made Snapchat so popular.

Just as odd as it is to be taking Facebook down a peg, perhaps it’s equally strange to see Snapchat enjoying a moment in the sun. It’s been virtually nothing but bad news for Evan Spiegel since he dared to take Snap public, particularly with regard to tepid audience growth.

Facebook and Snapchat are two sides of the same coin. On one side, you have this massive big-tent daily attraction for all ages. On the other side, you have one that is laser-focused on attracting the target demographic but that comes at the expense of a larger audience, thus impacting topline growth.

Their differing fortunes raises a question: Can a social platform be all things to all people with equal power?

There’s a rather pat, but nonetheless likely true, conventional wisdom explaining why Facebook could be weakening with a demographic so important to advertisers: kids don’t want to hang out with their parents, they want a place of their own. That’s true in real life and virtual life.

Worst-case scenario: If it’s not careful, Facebook could find itself in the same place as the pay-TV industry: a pretty massive audience seeing softness in such an important demographic that it calls into question the future growth of the platform.

You will know how seriously Facebook takes this by seeing what if any changes hit the platform in the coming months. If this is a big deal, perhaps features deemed sticky with younger auds will become more prominent. And if there is no noticeable difference, well, that tells you how concerned Zuckerberg really is.