Pinterest Sharing Buttons Hit Amazon and eBay

Fast-growing social curation site Pinterest has hit another online milestone: Amazon.com and eBay have added buttons to their sites that allow users to easily share anything from vintage lunch boxes to power tools on their virtual pinboards.

The additions make Pinterest the fourth sharing button on the sites in addition to e-mail, Twitter, and Facebook.

Pinterest is already the third most popular social network behind Facebook and Twitter, according to a recent report from marketing company Experian.