Dropbox used to say they use Amazon S3 [1], but now they just say "storage servers are provided by a managed service provider" [2] which could mean they found someone cheaper than S3, or that another company is managing the infrastructure for Dropbox. I always imagined Dropbox would move on from S3, and Drew even said back in 2010 that "We may not be on s3 forever, and will build our own store in addition, but for now this lets us focus on both the client software and the performance of the layer we’ve developed on top of s3." [3]

1. https://web.archive.org/web/20140101095459/https://www.dropb...

2. https://www.dropbox.com/help/7

3. http://www.makeuseof.com/tag/dropbox-review-invites-and-7-qu...