The reduction in ebook purchases may not be quite what it seems. There's no call to buy an ebook in advance of the moment you're ready to read it, unlike a paper book. Unless you might forget it entirely, of course. So once you've accumulated a fair bit of ereading, you're not going to buy anything more unless you know you're going to read it immediately.
Additionally, the fact that you can't actually turn the Amazon kindle's light off completely means I don't eread in the evenings; pity.