Amelia Brazell tells the story of sending a Twitter message to someone about the curative effects of an over the counter cold remedy called Zicam. A bit later, she received via Twitter a coupon for Zicam.
It's an interesting example of how new communications tools are changing advertising. A simple Twitter search by the Zicam folks allowed them to identify an individual customer and then at virtually no cost, send that customer a coupon. Try that with TV, radio, or magazine advertising.
It also demonstrates that using tools like Twitter may require some circumspection, as anything you share on Twitter is available to anyone via the Twitter search tools. Some Facebook users are finding out, to their chagrin, that posting certain kinds of intimate life details to virtual "reality" of Facebook sometimes has concrete (not virtual) consequences in the real world.