by Zoë Pollock
Though figures are still low and users too frugal, the rate of consumer purchases is growing remarkably fast. It's been nine years since the iPod's release, and already music downloads have shot up to the most popular user purchase. In just three years since the Kindle came out, e-books have stretched to 10% of users and ballooned to a billion-dollar market. And in only two years since Apple's app store went live, smartphone and tablet apps are now purchased by a fifth of all Internet users.