You know you should be spending some of your marketing budget on mobile ads, but what platform? The top two may not be unexpected: In 2013 Google mobile revenues will be up 92.1 percent to $8.85 billion, while Facebook’s mobile revenue is expected to increase by more than 333 percent to just over $2 billion. Twitter’s mobile ad revenue is also expected to increase this year to about 3.6 percent of total ad revenue in the U.S. For more on Google mobile ads, check Google’s Think Insights for help.