American Express is running yet another promo with the social network, Twitter. This time, when you sync your Twitter account with your American Express card, then tweet using a specific merchant’s promo hashtag, you’ll be eligible to earn statement credits at a variety of retailers, such as Whole Foods, Best Buy or McDonald’s.

Here’s how it works:

1. If you don’t already have a Twitter account, create one at twitter.com/signup.
2. Sync your Twitter account with your American Express card at sync.americanexpress.com/twitter (You’ll need to sign into your Twitter account and then enter your American Express card number)
3. Tweet the specified hashtag (see table below) of the promo you wish to enroll in. You may include as many hashtags as you want in a single tweet.
4. You can confirm that you’ve enrolled in the offer by clicking the “@ Connect” link at the top of the page (next to the Home link). It should show sync notifications from @AmexSync.
5. Complete your eligible purchase using your registered American Express card, per the individual offer’s terms.
6. You should receive a notification after you’ve completed your eligible purchase. While statement credits are usually posted within 2-3 business days of purchase, they may take up to 2 billing cycles to post to your account.

List of participating merchants

Be sure to read the specific offer’s terms which can be found at this link. Some promos require an in-store purchase while others may be online only. On certain offers, quantities are limited, so read carefully.

Edit 3/9/12: As part of South by Southwest, Amex is now also offering a $10 statement credit after you make any purchase using your Amex in Austin, Texas. Tweet #AmexAustin10 to register. For details, see aexp.co/Cmw.