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This just in: Facebook (NASDAQ:FB) is crushing it in mobile. OK, that might not be news to you if you've been even remotely following the social network over the past couple of years. Ever since it started trying in earnest to monetize mobile usage of its service, mobile ad sales have skyrocketed. Mobile usage has soared, and Facebook just said in its third-quarter earnings report that mobile-only users have now reached 1 billion for the first time ever. That's a huge milestone worth celebrating.

Two charts for the price of one

A couple years ago, Facebook began disclosing what percentage of ad revenue was coming from mobile ads. This started out at a paltry 3% of ad sales in Q2 2012. Last quarter, it was 84%. If you wanted to track this percentage over time, this is what it would look like:

Fb Mobile Perc

Data source: SEC filings. Chart by author.

As impressive as this chart is, it's not the one I'm referring to in the headline. Stay with me for a minute, since that chart doesn't paint the whole picture. Of course, total ad sales have been rising steadily over the years, too, so that rising percentage is compounded with growing revenue to become a very large business in absolute terms. On a trailing-12-month basis, Facebook has now generated $19.7 billion in mobile ad revenue.

If we take the inverse of that percentage, we can calculate Facebook's desktop-based ad revenue in dollar terms as well. Here's where it gets interesting, when we compare desktop and mobile revenue in absolute dollars.

Fb Mobile Chart

Data source: SEC filings. Chart by author.

When you look at it like this, it becomes clear that desktop-based ad revenue is fairly stagnant, albeit stable. Revenue fluctuates within a predictable range, but can now be viewed as an incremental side business on top of the core mobile ad business. Put another way, Facebook has generated $4.1 billion in desktop ad revenue on a trailing-12-month basis. The social network did $5.7 billion in mobile ad revenue last quarter alone. Pretty incredible if you ask me.

Evan Niu, CFA owns shares of Facebook. The Motley Fool owns shares of and recommends Facebook. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.