Todd told me that they started using Amazon S3 way back in 2006 to store uploads and attachments. They added CloudFront support in a matter of days, and also use the Amazon Mechanical Turk as part of their Smartsourcing feature:
PS - It turns out that both Todd and I have our own wood-fired pizza ovens, and we enjoyed chatting about the amazing pizzas that we can make! As a guest on The AWS Report, you are eligible for an invite to a future pizza party at my home. Contact us at firstname.lastname@example.org if you'd like to learn more.