Twitter's long been associated with the number 140; it is, of course, this number that defines how the site's users communicate with each other. And the number now represents the size of the company's payroll, too, as Twitter's hired its 140th employee. Biz Stone announced the milestone late yesterday by simply tweet ing, "Today we are celebrating our 140th employee at Twitter!" No details about whether the newest staff member is a janitor, programming whiz,