FORTUNE — More people are using Facebook. And they’re using Facebook more. The social networking site’s number of monthly users has jumped 21% year over year to 1.5 billion in June; 61% of those people logged on every single day. Not surprising then that, as Facebook’s robust ads business gains momentum, the firm’s stock price has finally shot above its $38 initial public offering debut.
But what’s going on, then, with teens? For the better part of 2013, reports have been rippling through the media that fickle youngsters are deserting the service. These reports trickle up through anecdotes as analysts, editors, and engineers study their children’s Facebook (SCOR) behavior.
CEO Mark Zuckerberg denies it. “Based on our data, that just isn’t true,” he said during last week’s earnings call. He explained the service has fully penetrated the U.S. teen market, and that the number of teens logging on to Facebook both daily and monthly has been steady over the past year and a half. Though he didn’t address whether the amount of time they spend on the service has leveled off or decreased, he said teens “remain really highly engaged.”
It’s hard to get an accurate read on what teens are really doing on Facebook. Outside data sources are particularly unreliable in their estimates for teen Internet behavior. For one, Comscore’s (SCOR) tool to measure mobile Internet activity — where a lot of Facebook’s most active users connect to the service — only measures users age 18 and over. And Facebook itself doesn’t break out its users’ numbers by demographics.
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I asked Zuckerberg about teen behavior directly during an April interview, and he explained his theory that social networking is not a zero-sum game. Teens may gravitate to Snapchat and embrace messaging apps like Kik and MessageMe, but they’re using Facebook more at the same time.
The most likely situation is that Facebook is just as populated, but it’s no longer popular. For years, the service was the backdrop for every cool digital party among teens. Upon turning 13 (and often well before), they set up an account and spent hours posting photos and videos for each other from laptops. But more recently, teens report feeling overwhelmed by the stress of reputation management on the site. A May survey of 802 teens released by the Pew Internet & American Life Project showed that many have lost enthusiasm for Facebook because, among other reasons, there are too many adults on the site and they feel “drained by the ‘drama’ they described as happening frequently on the site.”
For spent teens, there are a growing number of new tools like smartphones and tablets and an onslaught of new social services and games from which to entertain — and be entertained by — each other. As my 19 year-old breakfast companion said to me this morning, “What, you haven’t played Candy Crush yet? That’s the thing right now.”
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Of course, she hasn’t gone off Facebook and neither have any of her friends. In this context, Facebook has become the digital book of record for her life, the platform from which she can do other things (like, for example, play Candy Crush). It’s an easy way to get in contact with the kids from camp last summer, an old sixth-grade teacher, an aunt. Maybe she cross-posts from other social networking services like shopping site Wanelo. Probably, she cleaned up her language on it when it came to apply to college. “If I want to find someone, I text them, and if I don’t have their number, I Facebook them,” she explained to me.
In short, for teens — Facebook’s very first market and a market in which it has near dominance — Facebook has fulfilled Zuckerberg’s vision for the service; it has become a utility. While teens might play less on it, they turn to it more often. Once everyone is there — and in the United States, 94% of teens on social media are active on Facebook according to the Pew research — the cost of switching a network of relationships to a new service is too high.
Prolonged steady teen attention won’t be enough for Facebook in the long run. The company must continue to increase the time teens spend on the site, and so it will need to invent more things for them to do on and with Facebook. But it already has a jump on this endeavor. Photosharing service Instagram has seen monthly active users jump 600% to 130 million since Facebook purchased it less than a year ago. And as my breakfast companion explained to me, “Instagram is cool.”
