Apple basically created the tablet category in 2010 - since then continously loosing market share (from 95% to 25% in the 3rd. Quarter of 2014).
However, I think it is worth putting these numbers and the doomed iPad (if you believe some reports) into perspective. To do this here is the absolute numbers of tablet shipments (from Statista)
Apple owned the market in Q2 2010 but shipped only 3 million units. In Q2 2014 it held 25% of 49.3 million units - which results in sales of 12.5 million units compared to 3 million 4 years ago.
I believe the iPad is doing quite well and has a bright future - and competition from Android and Microsoft will only result in a better device from Apple in the end.