I saw an interesting graph posted by Sam Decker - CMO of Bazaarvoice at the Bazaarblog regarding Amazon's growth and overall retail market share trends. The graph below shows Amazon growth in % as essentially the mirror opposite of overall retail e-commerce. Clearly Amazon's customer base, email marketing, and overall customer experience continue to grow the business. However, I would offer that this chart is heavily related to the growth of another company that provides traffic to Amazon. Anyone want to throw out one guess who I am talking about? Yes of course, Google again. If you search on most retail product-oriented terms on Google, there is an extremely high likelihood that Amazon will show on page 1 (ie. my camera the "canon eos rebel" - Amazon shows in the 3 & 4 position on my search). The chart showing Amazon's growth continuing to march upwards over the last 6 years is roughly the same trend that Google's traffic has been on. If Google were to lose share or even decide not to rank Amazon as well any more, I would be willing to bet this curve would take a right turn quickly.
I am sure Amazon knows this quite well and probably sends flowers to the Googleplex often.
I searched the Hitwise blog to see if I could find a % of upstream traffic from Google to Amazon with no luck. Maybe someone else can find it?