Mobile ad revenue is a huge business, and it is expanding quicker than some can keep up with. Billions of dollars per year are generated, but who is at the top of the list? Google, that's who.

Online market research firm, eMarketer, have put estimates out which show that Google would make $1.42 billion this year. Another interesting fact is that the two social networking juggernauts, Twitter and Facebook, are making some big figures, too. Twitter are estimated to make more money from ad revenue this year, looking at raking in $129.7 million, with Facebook sitting back with $72.7 million.

Next year is a different question, where the tables are reversed, considerably. Twitter are estimated to make $272.6 million, whilst Facebook zooms ahead with $387 million. Google's estimated 2013 numbers? A whopping $2.38 billion. We do have some other players, Pandora, Millennial Media and Apple's iAd's, making $226.4 million, $84.1 million and $75.1 million, respectively.