It's occurred to me that the ultimate answer is embodied by the classic Feynman chapter title "You just ask them".
You've got Facebook raising money at over $50 per user based on the promise of futureperfect GMO (ie trying to imitate organic) targeted advertising. Google is so worried, they're gutting their core strength and namesake to become more 'social' by facilitating pop culture, as if it were hard to find. Clearly everyone on the web wishes they were buying things all the time, but just doesn't know it yet.
Meanwhile, Pinterest comes along and stumbles into creating a community where people curate pictures, many of stuff they desire to buy either now or in the future - inherently social status symbols. They skip ads, double-down on their ability to drive sales, and directly monetize affiliate links.
It seems like "Fail fast" has a corollary of "Context is king" - If you end up with all of the cat-picture-posting users in the world, and that's what users view your site as being for, you're never going to have better monetization than generic ads.