I'm not sure the popularity of the iPad is actually as high as everyone thinks. I think that's the Apple marketing reality distortion field. I see WAY more Kindles and Android phones than I see iPhones and iPads on trains in and out of London where I live.
http://mashable.com/2011/06/02/ipad-web-usage/
Your certainty that reality is otherwise seems to be in want of a citation.
Define "competitor" and "popularity". It's all damn lies and statistics.
My old Windows tablet could be a competitor. Dell do a tablet Windows machine. Motorola do a hybrid device. Is an ebook reader a tablet? Is an iPod touch a tablet?
Perhaps people with iPads hammer web sites? Perhaps people without iPads don't? Perhaps some mobile browsers identify themselves as desktop ones?
There is no defined taxonomy to classify things in either.
It's all pirates vs global warming.