Josh Davis:

If you post a pin to Pinterest, and it links to an ecommerce site that happens to have an affiliate program, Pinterest modifies the link to add their own affiliate tracking code. If someone clicks through the picture from Pinterest and makes a purchase, Pinterest gets paid. They don’t have any disclosure of this link modification on their site, and so far, while it has been written about, no major news outlet has picked up on the practice or its implications.

I use Pinterest and this is troubling. Yes, they need to make money but doing it behind the users back is a bit off putting.