Will Facebook adapt to mobile or will mobile adapt to Facebook?

We all know Facebook needs to become a force in mobile advertising, but just how much of a force? If Facebook were to replicate the success of its desktop ad business on mobile it would account for a healthy chunk of the entire world’s current mobile ad spend. And if it sticks with its planned approach to mobile ads it will need to create a whole new mobile advertising sector from scratch.

If Facebook’s future is mobile, it may not be enough for it to merely secure a piece of the mobile ad market: It might need to make the mobile ad industry figure out how to work with Facebook.

Berg Insight calculated that global spending on mobile advertising was $3.4 billion in 2010. And while it’s growing at rate of 37 percent per year, Facebook’s total revenues in 2011 were $3.7 billion, of which $3.1 billion came from ads. Meanwhile more than half of its 845 million monthly active users accessed the social network through mobile apps or its mobile website, both of which sport no ads.

Of course, most customers are using both platforms, not one or the other, so Facebook doesn’t need to build a $3 billion mobile ad business overnight. But even if Facebook were to replicate a fraction of its Web-based ad business on mobile, it would account for a large portion of the money currently being spent on mobile ads.

Mobile hasn’t been unkind to Facebook

Facebook isn’t losing money on mobile today. Far from it. EMarketer principal analyst Noah Elkin pointed out that 90 percent of Facebook’s customers are crossing between platforms, meaning they’re generating ad revenue in one way or another. They may not see any ads when they’re accessing the network from their phones, but they’re certainly seeing them when they log in from their Web browsers. One of the reasons those customers remain active and loyal to Facebook is because they have the flexibility to network socially from anywhere, so you could argue that mobile drives more ad views, not less.

It’s the remaining 10 percent, or 83 million users, that Facebook is worried about, Elkin said. Facebook doesn’t make a dime in ad sales off them, and while they may be a small segment of Facebook’s customers now, they’re its fastest-growing segment, as they are coming from the developing markets where Facebook’s future growth lies.

“The big concern is the mobile-only audience is growing faster than the multi-channel or desktop only audience,” Elkin said. “The long-term concern is that the mobile-only audience will come to dominate the social network.”

Enter the sponsored story

Facebook’s answer isn’t the display or search advertising responsible for the lion’s share of the mobile ad market today but the sponsored story, which it only recently introduced on the desktop interface and now plans to migrate to mobile. The format is so nascent that it’s not even really factoring into eMarketer’s current ad spend figures for traditional online spending, to say nothing of mobile.

Elkin said the format is promising not only because Facebook can use it to tailor ad content specifically to its users’ interests — which it knows a lot about — but also because it could factor in location and presence data unique to mobile. That means more value for advertisers and more revenue per ad. Ultimately that could overcome the limitations of mobile, where a small screen gets cluttered pretty easily. But it’s also an untried advertising format, and as my colleague Mathew Ingram points out, not even Facebook is convinced sponsored stories will work.

“If Facebook is successful, one of the impacts will be that it will grow mobile advertising overall,” Elkin said. Google(s goog), Apple(s aapl) and Millennial Marketing wouldn’t stop growing in the mobile ad space, but Facebook’s mobile ad revenue would be additive. In fact, one of the reasons eMarketer is projecting such huge growth in U.S. mobile advertising — jumping from $1.45 billion in 2011 to $10.8 billion in 2016 — is because it projects Facebook will precipitate a surge in new ads starting in 2014. If Facebook flops, then eMarketer will have to revisit its growth numbers, Elkin said.

Don’t piss off your future customers

Facebook faces some daunting tasks. It can’t just build a mobile ad business. It has to build the mother of all mobile ad businesses. And it has to do so with an ad format that’s still unproven.

Ultimately, its entrenched customers will cut Facebook some slack. They might gripe about ads crowding the limited real estate of their phones, but they’re not likely to abandon Facebook, considering how much of their online lives are embodied in the social network. But then again, those aren’t the customers Facebook is losing money on.

It’s the mobile-only users just discovering Facebook in India and other far-flung countries across the globe that the company needs to please. For many of them their first and only experiences with Facebook will be through mobile phones. If that experience isn’t engaging (and as my colleague Kevin Tofel writes, the Facebook mobile app experience is often subpar) then customers may abandon the social network entirely.