San Francisco - Twitter Inc., seeking a bigger slice of the more than $15 billion auto-advertising market, said households with users of its service were twice as likely to buy a new car as the average US household. About 6 percent of households with Twitter users bought a new car during a six-month period measured by partner Datalogix Inc. last year, the San Francisco-based social network said today in a blog post. Those households bought new vehicles at double the national rate from April to October, according to Datalogix and IHS Automotive's Polk. The car-buying data "moves the conversation from 'why Twitter?' to 'how should we use Twitter?'" Jeffrey Graham, ad research director at Twitter, said in a telephone interview. "The measurement shows that Twitter works and there's a huge opportunity there."