…It’s because they’re not on Facebook!

Pretty easy math on that one, right?

While much has been made this week about the legions of people on Facebook who have resisted clicking on ads, the Associated Press located the opposite pole of that news and dashed headlong into that direction to talk to some of the people who have managed to fend off the hype to join the social networking site that is steadily colonizing the world.

Several of the reasons given for why these hold-outs haven’t gulped down the Facebook Kool-Aid recall a lot of the complaints that the 901 million Facebookers frequently make about their experience on the website: no desire to rekindle decade-dead relationships, kind of boring, it’s a distraction, email and cell phones work just fine for communication, and so on.

Although most of the individuals profiled in the AP report might be described as the edge of Facebook’s bread and butter age demographic, it does highlight Facebook’s possible problem of appealing to a broader audience. MaLi Arwood, 47, said that she’s “absolutely in touch with everyone” she needs to be in touch with and therefore has no need for Facebook. Echoing a sentiment that nearly every Facebook member has bemoaned at least once, Arwood added, “I don’t need to share triviality with someone that I might have known for six months 12 years ago.”

While it’s impressive that only 2 of every 5 Americans haven’t joined Facebook, the fact that Facebook is most attractive to the a notoriously capricious age demographic doesn’t exactly lend any credence to the company’s long-term sustainability. Just ask Zynga, who purchased the mobile app, Draw Something, about how easily people can lose interest in a product. Or maybe revisit Rupert Murdoch’s lament about the demise of the once-popular MySpace. Permanence is an illusion.

That’s not to say that Facebook’s going to swerve off a cliff the way Draw Something appears to have done or be mismanaged the way MySpace was handled; far from it, really. Facebook’s got enough momentum to carry it for a while longer, but if the questions evoked this week about its inability to generate more revenue have any validity then the speculation that the company’s IPO will invite another disastrous tech bubble could have some substance.

Unless, that is, Facebook figures out some way to supplant the utility of extant, reliable ways to stay in touch like phones, email, and text messaging in order to appeal to the huge chunk of the population that currently doesn’t see Facebook as necessary.

More than that, Facebook will be required to achieve what no other tech company has been able to do so far: create a universal product that piques the interest of everybody across all demographic spectrums. In the absence of such innovation, there will continue to be more and more people who regard Facebook as an ephemeral commodity.