You should do some google digging on early Facebook, Twitter, and Amazon commentary. Granted, Facebook and Twitter don't have Google-level revenues, but early commentary about them was pretty much "they CAN'T make money on a social product". Facebook made over 4
billion dollars in 2011. Amazon's first years had some great commentary as well.
It turns out that EVERY SINGLE TIME someone builds a product that zillions of people love in an enduring way, they are able to hire world-class value-extraction experts who can make zillions of dollars.
Pinterest has the extra advantage of intent-- shoppers pinning products that they want/love gives Pinterest a lot more power to advertise cleverly than Facebook/Twitter. Unsure about Snapchat and RapGenius, but the key question is "With that audience and that growth, what could the smartest people in the industry turn them into if they were armed with functionally infinite spending money?"