Last December, Twitter quietly began testing a new self-serve advertising platform. This week, the microblogging service announced that it’s now accepting sign-ups from small businesses.

Previously, advertisers had to work with a sales representative to purchase ads — such as Promoted Tweets — but self-serve will enable you to buy ads directly from the website with only a credit card.

Through a promotion with American Express, Twitter is offering a $100 credit for free advertising to the first 10,000 businesses that register. The only catch is that you have to accept AmEx cards or be a cardholder yourself.

“So many hundreds of thousands and even millions of small businesses have been using Twitter effectively for years already,” Twitter CEO Dick Costolo told AdAge.

Through the use of Twitter’s free accounts, many businesses have succeeded in building loyal communities; However, self-serve makes it easier for small businesses to compete with larger brands.

According to Costolo, national brands tend to bid on keywords or hashtags associated with major events — such as the Super Bowl — which can make bidding competitive and expensive. Small businesses are more likely to bid on specific and localized terms.

The initial roll-out of self-serve had fewer than 20 advertisers and has since expanded to about 100. Twitter currently works with about 3,000 advertisers in total and that number is expected to grow significantly once the self-serve platform goes live in March.

If you’re interested in additional advertising options on Twitter, you can register on the company’s sign-up page.

[Via: AdAge, Image credit: Andreas Eldh]