iPad growth has slowed as it was originally designed as a high end device, so most people who would buy one have bought theirs (same with those replacing computers with them). It's hard to justify spending the money on something that only offers a slight portability upgrade on a MacBook Air unless you do something with it unique, like read comics.

The interesting piece is overall tablet use growing, as the non-iPad options continue to improve and offer a cheap alternative for doing things like going to a coffee shop or browsing on CalTrain.