By Robert Hof

You can argue until you’re blue in the face whether or not Snapchat is worth the $3 billion Facebook FB +4.52% apparently offered to buy it. But there’s little argument over why Facebook CEO Mark Zuckerberg was willing to pay that much for the disappearing-photo-sharing service: It’s attracting the teens that Facebook is losing.

At least, that’s the conventional wisdom. But here’s the problem: If there’s anything more ephemeral than Snapchat snaps, it’s teen attention spans. Today, Snapchat looks unbeatable, at least for what still seems like a rather narrow slice of social activity. But there’s no reason to think that teens will stick with most any app or service for long–all the less so when it seems that there’s a new hot social networking app every month or so these days.

So I’m betting Zuckerberg is a little smarter than that. What he really wants more than just a surge of new teen blood–as he also showed with his $1 billion purchase of Instagram–is to make sure that Facebook owns the most popular and compelling kinds of social networking as they develop. Snapchat clearly appeals to those who want to exchange bits of themselves in a more ephemeral way than they do on Facebook.