Twitter, the rapidly expanding microblogging service, plans to have hundreds of advertisers using its new ad system in the fourth quarter as the company ramps up plans to become a self-sustaining, profitable business. Chief Operating Officer Dick Costolo said the company’s newly-launched advertising system would provide a key pillar in Twitter’s plans to turns its popular service into a significant money-making operation that can justify its rich valuation. Costolo said he believed the company will eventually become profitable. But he declined to provide a timeframe for achieving profitability, which the company is currently hoping to generate from its advertising system and a forthcoming commercial accounts business, due to launch in July or August. The company is currently in the process of adding roughly another dozen advertisers to its so-called “Promoted Tweets” program which it unveiled with five participating advertisers last month, Costolo said. By the fourth quarter, Costolo expects hundreds of advertisers.