In a defensive move, AMR Corp.'s American Airlines in April "registered every possible Twitter name that could be associated with us," a spokesman says. The move came after airline employees last summer found a rogue profile in the name AmericanAir, which was shut down four weeks later.

Twitter co-founder Biz Stone says the company hopes to introduce a verification service later this summer to confirm profiles of public officials, public agencies and other well-known individuals. But he says there are currently no plans to offer an authentication service for businesses because of the costs and time that would be required.

Twitter policy prohibits unauthorized use of trademarks, but Mr. Stone says the company has no system for identifying violators. He says the service is working to respond to complaints within 24 hours, instead of the current average of five days.

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