Despite the iPad's disappointing recent sales figures, Apple remains the number one tablet brand by a wide margin.
Even though Apple's competitors, Samsung in particular, have recently narrowed the gap in terms of shipments, Apple's installed base (the company sold more than 200 million iPads since its introduction in June 2010) is still far greater than that of any of the company's competitors.
Data recently published by online ad network Chitika nicely illustrates how big Apple's lead in the tablet market still is. Chitika tracks tablet usage based on tablet-based impressions in its ad network and no competing brand even comes close to Apple in that regard. In April 2014, iPads accounted for 77% of tablet-based web usage in North America, dwarfing the share of Samsung and Amazon at 8% and 6% respectively.
You will find more statistics at Statista