At Scribd we initially used EC2 for systems like converting uploaded documents to HTML5 (the capacity we needed varied a lot over short time spans due to things

At Scribd we initially used EC2 for systems like converting uploaded documents to HTML5 (the capacity we needed varied a lot over short time spans due to things like variable API usage). Meanwhile, the app and web servers (and many others) lived at Softlayer (still the "best-looking horse at the glue factory" for managed hosting, IMHO).