Twitter Inc. has officially introduced a service which will permit small businesses to buy and place ads on the online messaging platform.

In a new promotion with American Express, Twitter will let AmEx cardholders and merchants who accept AmEx buy ads on its site. American Express will supposedly give each buyer a $100 advertising credit to get started by placing ads on promoted tweets and accounts.

Twitter seems to be taking notes from Google and Facebook, both of which also launched similar advertising platforms.

For months Twitter has sought ways to enlarge its advertising revenue and is hoping that the new ad service will to some extent resolve doubts about its long-term business strategy. In December, Twitter announced it would start testing self-service ads with about 100 advertisers. Now it will let in another 10,000, via a promotion with American Express.

Ads for the first group of 10,000 businesses will go live on Twitter in March, according to Ad Age. Twitter's ad revenue was $139.5 million in 2011 and is expected to grow to $259.9 million this year according to Emarketer.

Founded in 2007, Twitter allows people to send 140-character messages, or tweets, to groups of followers. The company has more than 100 million active users and a valuation of over $8 billion.