In general you don't want to delete absolutely everything. For example, usernames should not be reused, so you can't "delete" them -- you can tombstone them though, and you should. Besides tombstoning to prevent reuse, you can and should delete as much associated metadata as you're willing to / contractually or legally required, naturally.

Even what you can delete can (and will) survive in logs and backups, web archives, screenshots, etc. Deleting things on the Internet is just difficult.