Despite Android's dominant lead in both the smartphone and the tablet market and an installed base that likely exceeds 2 billion devices, Apple's mobile platform iOS used to have the edge in terms of mobile web usage until very recently.
According to data collected by NetMarketShare, a company that tracks usage across a network of more than 40,000 websites worldwide, it wasn't until July 2014 that Android finally surpassed iOS in terms of web traffic caused by its users. Since then, Android has gradually built its lead. In April 2015, Android devices accounted for more than half of global mobile web traffic for the first time, while Apple's share dipped to an all-time low of 38.8 percent. Meanwhile all other mobile platforms are rendered all but irrelevant with Windows Phone and the discontinued Symbian hovering between 2 and 3 percent share of mobile web browsing.
This chart compares Android and iOS in terms of their share of mobile web browsing.
You will find more statistics at Statista