There are several reasons for that, and some of them I'm sharing below: 1. The paid ads on Pinterest have high ROI because you only have to pay when someone clicks on your Pin. 2. The content on Pinterest never gets old it stays forever so if your pins are really valuable then even after years they will be shown to users. 3. The user base of Pinterest is highly engaging, if people see that something that they don't want to miss they will pin it immediately. 4. With Pinterest you don't have to look for common interest pattern in people, you can post something unique and even that will get quick attention.