Unless they are using the really big instances though, I find that EC2 nodes are generally lower in power compared to what most people are using for their servers.
I remember not too long ago before they moved to EC2 everything fit in just a few racks. Of course, their traffic has expanded significantly recently.
EC2 nodes are terribly underpowered, particularly in the I/O realm (the biggest weakness of virtually every site) where a standard desktop absolutely annihilates the I/O performance of an S3 connection (where you have to perform software RAID to get anywhere near the performance of a single magnetic low-end SATA drive, which is absolutely terrible).
A lot of the "web scale" noise has come from dealing with these gerbil sized instances.
I remember not too long ago before they moved to EC2 everything fit in just a few racks. Of course, their traffic has expanded significantly recently.