Check out this article How do you delete a file on the cloud? from Stephen Baker. After my experience of working on web applications I can attest to the fact that nothing is deleted. Deleting a record from a database causes all kinds of performance and maintenance issues. It's the equivalent of removing a brick from a wall. Remove too many bricks and the wall integrity is compromised. So instead of deleting you account, email or post, your data is simply flagged as deleted.
The moral of the story: Be careful what you put out there because it's there for good.