Baremetrics (https://www.baremetrics.io) by Josh Pigford is built off of Stripe. There are also a handful of dunning-related services that sit on top of Stripe, also.
Gnip (http://gnip.com/) was just bought by Twitter. I would argue that they built their whole business on top of twitter & other social media companies.
Zynga was basically built on top of Facebook. Upworthy was built on link traffic from FB. Then FB changed the way they allowed / promoted sharing and both companies took a hit.
Search for jawed karims talk at CMU I guess. One of the key reasons of YouTube fast growth was the HTML snippet that you could embed in you MySpace page.
I remember MySpace ended up blocking YouTube from their site. And YouTube, a small startup at the time, turned to their users to plead to MySpace to activate them again. A few weeks later, they were back in business. But it's interesting to know how close YouTube was to hitting the deadpool.
They use aws as infrastructure but aren't inseparably welded to it. They could presumably switch to hosting elsewhere without changing their product itself.
You can see them as abstractions of AWS in some ways. An average consumer wouldn't go through the trouble of dealing with AWS to backup their files and even for developers heroku provides a nice level of abstraction
