Amazon.com depends on AWS but not all the services and not necessarily in the critical path of serving up pages on the main website.
E.g. All the servers run on EC2 but they don't really use ELB or EBS. S3 is used heavily throughout the company but some of the newer services not so much.
E.g. All the servers run on EC2 but they don't really use ELB or EBS. S3 is used heavily throughout the company but some of the newer services not so much.