With the rapid increase in smartphones & mobile browsing, mobile ad market has become a major cash cow for companies like Google, Facebook & others. Google is the mobile ad industry leader followed by facebook. Even though facebook was a late entrant to the mobile ad market, it has gained a major chunk of the mobile ad market. They are followed by twitter at the 3rd spot. With the prediction of more browsing on mobile than desktop in the coming years, companies are pushing new boundaries to capture the mobile audience.
You will find more statistics at Statista
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