The twitter bird logo often seen around the website
Twitter is a free social networking and micro-blogging service that enables its users to send and read each others' updates, known as tweets. Tweets are text-based posts of up to 140 characters, displayed on the author's profile page and delivered to other users - known as followers - who have subscribed to them. Senders can restrict delivery to those in their circle of friends or, by default, allow open access. Users can send and receive tweets via the Twitter website, Short Message Service (SMS) or external applications. The service is free over the Internet, but using SMS may incur phone service provider fees.
Since its creation in 2006 by Jack Dorsey, Twitter has gained notability and popularity worldwide. It is sometimes described as the "SMS of the Internet", as it provides the functionality—via its application programming interface (API)—for other desktop and web-based applications to send and receive short text messages, often obscuring the Twitter service itself.
Through SMS, users can communicate with Twitter through five gateway numbers: short codes for the United States, Canada, India, New Zealand, and an Isle of Man-based number for international use. There is also a short code in the United Kingdom which is only accessible to those in the Vodafone network.
Alexa ranks the site 27th in terms of web traffic. Estimates of the number of daily users vary, because the company does not release the number of active accounts. However, a February 2009 Compete.com blog entry ranked Twitter as the third most used social network, which puts the number of unique monthly visitors at roughly 6 million and the number of monthly visits at 55 million; however, only 40% of users are retained. In March 2009, a Nielsen.com blog ranked Twitter as the fastest-growing site in the Member Communities category for February 2009. Twitter had a monthly growth of 1382%, Zimbio of 240%, followed by Facebook with an increase of 228%.