In most situations, the person juggling e-mail, text messaging, Facebook and a meeting is [not multitasking, but] really doing something called “rapid toggling between tasks,” and is engaged in constant context switching.
As economics students know, switching involves costs. But how much? When a consumer switches banks, or a company switches suppliers, it’s relatively easy to count the added expense of the hassle of change. When your brain is switching tasks, the cost is harder to quantify.