American Express launches first "pay-by-tweet."

American Express (AXP) yesterday launched the first "pay-by-tweet" service on Twitter, enabling cardholders to make purchases by sending a short message. The offering represents one of the most ambitious attempts so far to build e-commerce directly into Twitter, although strangely, the social media site won't be taking a cut of the transaction revenue. Instead, the hope is the service will boost its ad income.