21% of Pinterest users have bought something they discovered on Pinterest, so it makes sense the big ecommerce websites would add "Pin This" buttons: http://techcrunch.com/2012/04/11/amazon-ebay-among-latest-to-add-pinterest-buttons/

I can remember a time not long ago when The Web was teeming with Digg and Delicious buttons.

Now it's all LIKE, TWEET, REBLOG, and PIN.

Sites need to start serving these dynamically based on referrer URL and past activity - if I arrive at eBay or Amazon via Pinterest, I should be shown a Pin button. If I arrive via Facebook, I should be shown a like button. Subsequently, if I have liked a crap ton of pages, but never pinned anything, stop showing me a pin button. There's a whole startup idea right there - tech to serve sharing buttons based on behavior and referrer.

I like that idea a lot, Brendan.

A site that adapts to its users makes for happy users.