Amazon Web Services is an ideal hosting environment for these applications. Developers can start small, test and prove out their ideas, and then rapidly add processing and storage resources as they develop an audience.
Amazon is talking about building Facebook Applications on AWS and you can visit a new page on Building Facebook Applications on the Amazon Web Services Platform, you can find step-by-step instructions for registration along with links to tutorials, and even a pre-built EC2 AMI.
Nice to see them working to promote and educate people on how to use the AWS Platform to inexpensively build highly scalable Facebook applications.