Social media marketers may be forced to grapple with this in a big way soon as the audience of the world’s largest social network, Facebook, changes.

As observed by CNET’s Jennifer Van Grove, the publicly-traded social networking behemoth made an interesting disclosure in its latest 10-K filing with the SEC. The filing detailed a new risk:

We believe that some of our users, particularly our younger users, are aware of and actively engaging with other products and services similar to, or as a substitute for, Facebook. For example, we believe that some of our users have reduced their engagement with Facebook in favor of increased engagement with other products and services such as Instagram.

While younger users — an elusive and often lucrative target for marketers — may not be abandoning Facebook like a party that has been shut down by the police, Facebook’s disclosure that “some” of them are starting to look for something better to do is particularly notable given the company’s stance on its staying power with this group less than a year ago.

In its IPO prospectus, Facebook shut down the notion that tech-savvy youngsters were leaving, telling prospective investors, “We also believe that younger users have higher levels of engagement with the web and mobile devices in general and with Facebook specifically.”

A problem for Facebook, a bigger problem for marketers

Obviously, an exodus of younger users would create problems for Facebook. These users, who are often the earliest adopters, have traditionally been the backbone of Facebook and as the most mobile-engaged demographic, they could throw a wrench in Facebook’s ongoing effort to monetize mobile users, which has been showing progress of late.

But if younger users start to flee Facebook, it could be an even bigger nightmare for social media marketers. After all, many of them will need to rethink what the world’s largest social network means to their brands and how the experiences they’ve created on Facebook might need to be altered as the demographic make-up of Facebook changes. In some cases, these marketers may find that their brands are positioned inappropriately. For instance, a brand that had assumed its initiatives on Facebook were reaching users between the ages of 18 and 25 might have to reconsider those initiatives if it turns out that they’re increasingly reaching older users.

Social media as a dynamic channel

Demographic shifts are not a new phenomenon, nor are they a complete mystery to marketers. As populations change, marketers notice and change their messages.

But digital channels, and social channels in particular, are capable of seeing significant shifts on much shorter time spans than most marketers are used to. Instead of these shifts tracking the gradual changes that take place within the general population at large as birth rates change and people age, for example, a shift seen on a service like Facebook can take place relatively quickly as the next big thing becomes the last big thing.

While it’s too soon to suggest that Facebook is undergoing an earth-shattering demographic shakeup, and it’s important to note that Facebook has not defined “younger users”, marketers must accept and embrace the fact that social media is inherently a dynamic channel. As such, it’s not unreasonable to expect that popular services will see major audience changes on the order of years, not decades, and in fact, they should plan for this.

That means preparing for the eventuality that a brand will need to reposition its presence on a popular social network, and in some cases, change who it’s targeting entirely. Perhaps more importantly, it also means assessing social investments with the understanding that they may not produce assets that grow predictably or linearly.