How does Facebook make money on mobile?

We all know why Google would develop Android and give it away. The more Android smartphones, the more searches and thus, the more advertisements.

What about Facebook? Does anyone have any reliable numbers on how much revenues Facebook generates:

mobile vs non-mobile?

Soon, I predict that most of the users (and usage) on Facebook will be via mobile. Can someone ask Mark Zuckerberg how Facebook plans to add new and transition existing sources of revenue from non-mobile to mobile? Thank you. Readers know I have long held Facebook's potential in high regard. That said, the company will be forced to go public by about 2012. That same year, or probably by about 2013, there will be a tipping point where most users access Facebook via mobile and when most of the Facebook usage is via mobile.

Is that enough time to create a competitive display ad network? Will there be more usage, but less depth of usage and, if so, will advertising, coupons, recommendations be less powerful? Without a Facebook smartphone, we will already have -- and be comfortable with -- non-Facebook payments platforms. We will play games but, again, without a Facebook phone, these will likely be more robust if we use a native app -- shutting out Facebook. Groupon and Foursquare will have location-based and time-based coupons and buying incentives covered. As we all get smartphones and all post our smartphone-based videos on the web, primarily Facebook (let's assume),theta represents a potential for revenues. Years in and YouTube still hasn't done a good job of monetizing user videos.

With a billion users I don't want to overstate the problem. Still, I hear Facebook execs regularly talking about the focus on optimizing the service on mobile. However, I primarily hear them talking about generating revenues on the non-mobile site.

(Note: Please let me know in advance if you wish anonymity.)