Your example (reddit using S3) is not a database use, it is a content delivery network use. For that, it is good enough, but I'd choose a different key-value store for database, and mostly because of the latency (been there, tried that).
Also, I would argue that static content delivery is just another form of database. It's just a massive key/value store, there the keys are the files and the values are the contents of the keys.
Let me ask you this: What is your definition of a database?
(1) any page-load that requires 3-5 sequential DB access (based on the results that are returned in the previous steps).
(2) Netflix's "let's store somewhere until we really want to use it" qualifies.