Facebook's purchase of the staunchly ad-free Whatsapp network certainly raises some interesting questions, and plays nicely into the growing narrative that Google's ads-everywhere culture has had its day.

“I don’t personally think that ads are the right way to monetize messaging systems,” Facebook CEO Mark Zuckerberg said today on a conference call about the acquisition.

Facebook certainly does a tidy business selling ads to display in its apps for iPhones, iPads, and Android devices. Still, the company faces tough challenges as it tries to grow from here. Mobile ads are still less valuable than the ones seen by Facebook users on their PCs. And simple geometry dictates that mobile devices have less space for advertising than the expansive desktop web.

But there’s something else, more fundamental: a disquieting suspicion that, in the long run, advertising simply might not work for the mobile web.