Ever since the iPad’s release in 2010, iOS used to be the no. 1 platform in the tablet market.
Until now. In 2013, the same thing that applied to the smartphone market a couple of years ago happened in the tablet market: many manufacturers build low-cost Android devices that gradually eat away at Apple’s market share.
As an individual manufacturer, Apple remains in the lead, but in terms of operating systems, iOS has no chance against the sheer volume of Android devices.
You will find more statistics at Statista