(W20, SN) Twitter is a social networking and microblogging service, owned and operated by Twitter Inc., which enables its users to send and read other users' messages called “tweets”. Tweets are text-based posts of up to 140 characters displayed on the user's profile page. Tweets are publicly visible by default, however senders can restrict message delivery to their friends list. Users may subscribe to other users' tweets—this is known as following and subscribers are known as “followers”. All users can send and receive tweets via the Twitter website, compatible external applications (such as for smartphones), or by Short Message Service (SMS) available in certain countries. While the service is free, accessing it through SMS may incur phone service provider fees. The website is owned by a company that is based in San Francisco, California. Since its creation in March 2006 and its launch in July 2006 by Jack Dorsey, Twitter has gained popularity worldwide and currently has more than 175 million users. It is estimated that Twitter has 190 million users, generating 65 million tweets a day and handling over 800,000 search queries per day. It is sometimes described as the "SMS of the Internet”