Twitter, which for years has sought ways to expand its advertising revenue, hopes the new ad service will at least partially answer looming questions about its long-term business strategy.

Since 2010, Twitter's in-house sales staff has sold "promoted tweets" to large businesses on a case-by-case basis. The company spent last year developing a self-serve system that could handle a far greater volume of ad transactions and, in November, opened the system to a small number of clients for testing.

Officially launched on Twitter's website on Thursday, the new service is limited to merchants and advertisers who use American Express cards. Twitter will open the service to other cardholders in the coming months.

The rollout comes at a critical phase in the company's development and will be closely observed by investors and analysts curious to see if the company will go public anytime soon.

Founded in 2007, the San Francisco-based company lets people send 140-character messages, or tweets, to groups of followers. The company has more than 100 million active users and a valuation topping $8 billion, even though it has not yet established a money-making model.

Co-founder Jack Dorsey said in January that Twitter's "business model has been in development for some time and it works.

(Reporting By Gerry Shih; Editing by Matt Driskill)

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