Quora explains using AWS for "everything" here: http://www.quora.com/What-stack-does-Quora-run-on-EC2, but they don't say how many of what. Their traffic is a secret too, despite what the article tries to assume of AppData, but I'm curious why it is believed that running application servers and their databases/cache is economical. Does Quora really see a huge flux in activity and inactivity?