American Express' twitter campaign generated an estimated 160,000 tweets and $7.2m in spend on the cards

"A campaign launched by American Express in the first week of March on Twitter has generated some eye-brow raising numbers.  Called Tweet Your Way to Savings, the campaign asks AmEx cardholders to tweet #AmEx hashtag in order to receive a deal the cardholder is marketing. The savings are automatically applied with the purchase.
Visibli Social Analytics, which has been tracking the #AmEx hashtags since the campaign launched on March 6, reports that the campaign has generated so far a total spend of approximately $7.2 million and discounts of $1.4 million, roughly. There have been 160,707 tweets, or approximately 259 tweets per hour."

Source:  MarketingVox, 16th March 2012