What are the biggest apps on the simplest architectures and what are the architectures?
Everything from StackOverflow to ad serving systems to high-frequency trading exchanges are run this way.
Quora, Pinterest, Twitter, etc are all just big app instances talking to DB instances, with separate systems for background processing, queues, and caching. Are you suggesting that they would scale better with serverless functions instead?
Here's a list of architectures: http://highscalability.com/all-time-favorites/