As the program is Twitter-based and most tweets are public, Visibli tracked all those #AmEx hashtags from the start on March 6. It reports that more than 25,000 AmEx U.S. cardholders have signed up and claimed more than $1.3 million worth of coupons. Visibli is publishing the Tweet tally in real time.

For those who haven't been following the AmEx chatter, here's how it works: Cardholders sync their accounts with their Twitter handle, and when they tweet with the specified hashtags, savings are automatically loaded onto their account when they use that card at the merchant.

AmEx launched this couponless offer program for cardholders to tweet, say, #AmexMcDonalds when spending at the fast-feeder for $5 loaded directly back to their credit-card account or #AmexZappos for $10 back on the next $75 or more spent at the e-tailer.

While they can tweet and save at merchants ranging from Whole Foods Market to H&M and Virgin America, so far the favorite coupon, measured by pure volume of #AmExMcDonalds tweets, has been for fast food. Whole Foods and Best Buy follow.

The program also appears to be a boon for Twitter, long criticized for the many people who sign up and never tweet. But it appears the AmEx program is attracting new and dormant Twitter users. Of the tweeters claiming offers, almost 28% are completely new to Twitter and 23% had signed up before but fallen inactive before using the hashtags, Visibli cofounder Saif Ajani told Ad Age .

It's likely less of a surprise that sign-ups peaked when AmEx hosted its Hova concert and live-streamed the performance on YouTube.

This is just the latest from AmEx in couponless offers that trade social-media mentions from its card-holders for savings. At SXSW last year, the company tested a program with Foursquare in which check-ins at local businesses would result in savings being automatically loaded to their synched credit-card accounts. The program was expanded during its national promotion Small Business Saturday .