There was a great answer on Quora –
80 million objects stored in S3 with 410 terabytes of user data, 10x what they had in August. EC2 instances have grown by 3x.
Pay for what you use saves money. Most traffic happens in the afternoons and evenings, so they reduce the number of instances at night by 40%. At peak traffic $52 an hour is spent on EC2 and at night, during off peak, the spend is as little as $15 an hour.
150 EC2 instances in the web tier.
90 instances for in-memory caching, which removes database load.
35 instances used for internal purposes.
70 master databases with a parallel set of backup databases in different regions around the world for redundancy.
Sharding is used, a database is split when it reaches 50% of capacity, allows easy growth and gives sufficient IO capacity.
Hadoop-based Elastic Map Reduce is used for data analysis and costs only a few hundred dollars a month.