If you haven't been using Twitter lately but don't want someone to grab your username, you might want to sign in shortly. Twitter is notifying users that it'll start removing accounts that haven't signed in for at least six months. It'll begin the purge after December 11th, although a Twitter spokesperson stressed that the account closures will take place over "many months" -- you won't see legions of usernames become available on December 12th. You don't have to post to keep an account, so don't worry about having to pretend to be active.