Twitter is an online social networking service that enables users to send and read short 140-character messages called 'tweets'. Registered users can read and post tweets, but unregistered users can onlyread them. Users access Twitter through the website interface, SMS, or mobile device app. The service was launched on July 2006. The service rapidly gained worldwide popularity, with more than 100 million users who in 2012 posted 340 million tweets per day. The service also handled 1.6 billion search queries per day. In 2013 Twitter was one of the ten most-visited websites, and has been described as 'the SMS of the Internet.' As of December 2014, Twitter has more than 500 million users, out of which more than 284 million are active users.