Actually they do warn you after the fact -- you should get a "welcome back" email: Hey k3n, The Facebook account associated with facebook@example.org

Actually they do warn you after the fact -- you should get a "welcome back" email: Hey k3n, The Facebook account associated with facebook@example.org was recently reactivated. If you were not the one who reactivated this account, please visit our Help Center (http://www.facebook.com/help/?topic=security). Thanks, The Facebook Team