First movers get rewarded... just ask Spotify on Facebook or Loopt on iOS. This of course assumes that you are a first mover on a big, impactful platform. There is little question Pinterest is both big and impactful. So Amazon is experimenting with it.
Here is a screenshot of the Pinterest button on an Amazon product page - beside email, Facebook and Twitter buttons. For e-commerce companies - Pinterest is arguably more relevant here than either Facebook or Twitter. It is probably most valuable to get a Facebook share - but users are probably far more inclined to post to Pinterest... and for many of those users, Pinterest will then post back to Facebook.
Fascinating to watch.