Dropbox launched support for webhooks. This is a great move.
Amazon S3 does have something similar. You can enable notifications via their Simple Notification Service ( SNS ), but then you are charged for the SNS usage. For many people that will make it less appealing than traditional direct webhooks.
Dropbox continues to use a mix of Amazon Web Services ( AWS ) and separate dedicated hardware to manage user file. I wonder if they are using any AWS services to process what are likely millions of HTTP requests for webhooks.