Regarding your related question, [the engine automatically deletes abandoned accounts][1].

> When is it safe to declare an account abandoned? We came up with these
> two rules of thumb. If..
> 
>  - the user has not visited the site in six months    AND
>  - the user has not done anything of significance, ever
> 
> .. their accounts are effectively abandoned. With that in mind:
> 
>  - if the user has not visited the site in six months   AND
>  
>  - the user has not done anything of significance, ever .. their accounts are effectively abandoned.
> 
> We delete cookie-based unregistered accounts when:
> 
> - The user has not visited Stack Overflow in six months   AND
> - The user has less than 50 reputation, and no visible (not-deleted) posts
> 
> We delete OpenID registered accounts when:
> 
> - The user has not visited Stack Overflow in six months   AND
> - The user has only 1 reputation, no visible posts, and no other accounts on the network


  [1]: http://blog.stackoverflow.com/2009/02/when-is-an-account-abandoned/