Twitter (News - Alert) is expanding its self-service advertising platform to include more small businesses in the United States through an exclusive deal with American Express (News - Alert). The new ad program, set to launch at the end of March, will be the latest step by the social networking platform to broaden its advertising revenue.

Twitter has a website allowing small businesses, merchants, and other companies looking to display ads on their social networking site to sign up and register for the program. The companies that sign up early could receive a bonus. As a promotion Twitter is offering a prize to some local businesses that might entice more users to sign up by offering $100 in free Twitter advertising, sponsored by American Express, to the first 10,000 eligible Cardmembers and merchants that register.

“This program is open to American Express Cardmembers and merchants who use Twitter to tweet business news and updates to their followers who are actively engage with and respond to Twitter users through mentions, retweets, and replies,” according to the terms posted.

Until now, advertisers or ad agencies had to go through a Twitter representative, which is limited access to ads. Twitter has said that selling ads to small businesses is one leg of its growth plan.

The ad-expansion plan comes as Twitter works to live up to a valuation of more than $8.4 billion, based on recent private investments. Despite those numbers, Twitter is still behind other social networking platforms such as Facebook and LinkedIn.

CEO Dick Costolo (News - Alert) said Twitter is not looking to go head to head with any other social networking company. Costolo said Twitter is just looking to expand its current advertising market. “I don’t look at what other companies are doing,” he said. “We don’t think in terms of building the company for a particular IPO date. We are trying to build this company for long term…I have every expectation that we will be able to scale this very rapidly,” said Costolo.