[krapp]

It's not actually true that everything anyone does online will be permanently archived. If it were, there would be no need for the Internet Archive.

The truth is, only the things someone has an interest in archiving will be archived, and only so long as someone has an interest in maintaining those archives. Just look at the recent announcement about Yahoo Groups... no one was, and likely no one is, going to permanently archive most of that. Sites, content and history get lost all the time.