WASHINGTON (AFP) - Twitter users are sending 200 million tweets a day, up from 65 million a year ago, the micro-blogging service said. "For context on the speed of Twitter's growth, in January of 2009, users sent two million tweets a day," Twitter said in a blog post. The San Francisco-based Twitter was founded in 2006. It has more than 200 million users. According to a recent study by the Pew Research Centre, 13 percent of the online US adults aged 18 and older use Twitter, up from eight percent in November 2010.