Agree with the author that transparency is the best policy when it comes to social media networks. Secrecy backfires later.
Pinterest is automatically swapping out the links behind product pins, using a third-party service called Skimlinks (a thriving business of its own), with its own affiliate links. A pin that points to a product on Amazon, for instance, will pass the clicker through to the product page with a Pinterest affiliate code thrown in for good measure. And should that person go on to make a purchase, Pinterest pockets the affiliate money.
Pinterest likely should disclose this practice to users even if they aren’t required to do so by law, if only to maintain trust with their users