Twitter announced on Tuesday that it has acquired BackType, a San Francisco-based startup that helps publishers measure the impact of their social media use. Financial details of the transaction were not released. Twitter, in a message on its @TwitterGlobalPR feed, said BackType would \"focus on creating tools for our publisher partners.\" BackType said it would be relocating to Twitter headquarters in San Francisco. \"Our vision at BackType has always been to help our customers understand the value of engagement on Twitter and other social platforms,\" BackType said in a message announcing the sale on the company website. \"Joining Twitter gives us the opportunity to bring insight to tens of millions of publishers around the world that are using Twitter to communicate and connect with their audience,\" the company said. Twitter, which was founded in 2006, announced last week that Twitter users are sending 200 million tweets a day, up from 65 million a day one year ago.