Dropbox stores their stuff in S3 a little different. It's not a 1:1 correspondence between user files and objects under Dropbox's S3 account. The fact that they use S3 as their backing store means very little. It certainly sounds good to have S3 in back when you talk about scalability and durability, but 1) they could just as easily use something else, and 2) depending on their sharding strategy, a single lost object could impact multiple files at the user level.