FORTUNE -- When Mark Zuckerberg introduced Facebook Home this month, he made it clear that he and his fellow executives seem to actually believe that people love Facebook, the brand, the same way some people love, say, Apple (aapl) -- so much so that they want to make it the center of their digital lives. But people don't love Facebook; they love interacting with family, friends, and acquaintances, and they put up with all of Facebook's many annoyances, frustrations, and privacy invasions in order to do that. Facebook just happens to be the platform that attracted a critical mass of people, which is the only thing that makes it valuable.
When that critical mass starts to fall apart, the company's value falls apart with it. And it's almost a guarantee that it will happen eventually, especially given that Facebook (fb) doesn't seem like it's going to stop making annoying changes to its interface or its privacy policies any time soon. If a report released Thursday by Piper Jaffray is accurate, the cracks are beginning to appear.
Teenagers are increasingly opting for other social-media experiences, the report concludes. While Facebook is still the top social network among teens, their use of the site has dropped by 9% since last year. Piper Jaffray says the main beneficiaries of Facebook's loss are Reddit and Twitter. And that seems to be true to an extent, but looking at the trends from a wider perspective, it seems like the growth of social media options is creating a kind of teen diaspora. The report also cites Vine, Snapchat, Kik, and 4chan as preferred destinations. Instagram remains popular, too, though it saw no growth in teen usage over the past year, according to the report. As if to highlight the unpredictability of teen behavior, Twitter use among teenagers over the past year fell by several percentage points, then recently rose again nearly to where it had been last spring.
Some observers have cited the presence on Facebook of parents, grandparents, and other boring, old adults as the reason for kids going elsewhere. But that's been the case for a few years now. It seems likely that the presence of parents, etc., is making it easier for kids to move on, but it's probably not the proximate reason for them doing so. Teenagers are both easily bored and attracted to new experiences. They would have moved on no matter what. It's actually possible that the presence of adults is one of the things that's keeping them on Facebook at all, albeit for less time. By this theory, they keep Facebook because that's where everybody they know (including their Aunt Gladys and Uncle Wayne) is hanging out. They go to Twitter and Instagram because that's where the people with whom they want to spend most of their time with -- their friends and like-minded others -- are hanging out.
The problem for Facebook as that the kids who are spending less time there might represent the beginning of a long-term trend. If that's the case, it might eventually spread to adults, too. And worse, younger kids might be less inclined than their slightly older cohorts to spend much time on Facebook, even if they maintain an account there.
There is nothing particularly special about Facebook's platform, or certainly about its brand. It succeeded thanks mainly to timing, and to the inherent terribleness and woeful mismanagement of MySpace. Facebook came along at the perfect time to amass a user base large enough for network effects to take over: People are on Facebook because people are on Facebook. But the service itself can be easily replicated, and if people -- especially teenagers -- think they can get the same value elsewhere, they will go elsewhere.