Affiliate links make money for your blog. If you belong to a vendor's affiliate program, you use a special affiliate code to link products to the vendor's ecommerce site, and you get money any time someone ends up buying a product using that unique link. Many sites (18,000 according to this report on Mashable) use a service called Skimlinks to automatically attach affiliate code to links that users leave on their sites. It isn't news that sites affiliate to generate revenue -- except that Pinterest has been doing so without disclosing the fact. (Note: BlogHer's interpretation of the FTC Guidelines for ethical disclosure indicates that when a blogger uses affiliate links, they should disclose that they do so.) Upon learning of the Pinterest/Skimlinks relationship, LLSocial wrote about it, causing some controversy. Here's a highlight from the post:
As most bloggers are aware, when you use an affiliate link in your post, you need to provide some type of disclosure either by it clearly being an ad, mentioning it is an affiliate link or at a minimum providing some type of prominent disclosure that your site features affiliate links. This is done, because you have a financial interest is promoting the product.
In Pinterest’s case, since they are not creating the content and are inserting the links automatically, they might feel that they are not promoting affiliate linked pins any more than other pins, and thus they don’t need to disclose as the placement is not affected based on the financial gain.
skimlinks own site has a FAQ section about disclosure, and it would seem their own recommendation would be that Pinterest make a disclosure.
Multicolored pins by Voronin76 via shutterstock.com