Only about half of Twitter's 200 million-plus registered members log on daily but the micro logging website is chalking up growth of 40% every quarter in mobile device usage, Chief Executive Dick Costolo said on Thursday. Twitter, one of a coterie of Internet social networking services like Facebook and Google Inc's embryonic "Google+", is gearing up for a hotly anticipated initial public offering. But Costolo told reporters they would do so only on their own terms. "We want to be able to remain independent, grow the business the way w e want to, and not be beholden to public markets until we feel like we want to be," Costolo said at Twitter's offices. Twitter, which allows people to send 140-character text messages, or Tweets, to groups of so-called followers, is one of the Web's most popular social networking services, along with Facebook and Zynga. The website has grown into a social phenomenon employed by celebrities and politicians alike, hailed at times for promoting the free flow of global information. Analysts have sought details however on how many of Twitter's registered users are "active", a key metric in determining its business potential. Twitter.com now sees about about 400 million unique visitors every month, a 60% leap from 200 million at the start of the year.