What is Twitter?

Twitter is a free social networking and micro-blogging service that enables its users to send and read other users' updates known as tweets. Tweets are text-based posts of up to 140 characters in length which are displayed on the user's profile page and delivered to other users who have subscribed to them (known as followers). Senders can restrict delivery to those in their circle of friends or, by default, allow anybody to access them. Users can send and receive tweets via the Twitter website, Short Message Service (SMS) or external applications. The service is free to use over the Internet, but using SMS may incur phone service provider fees.
Since its creation in 2006 by Jack Dorsey, Twitter has gained extensive notability and popularity worldwide. It is sometimes described as the "SMS of the Internet," in that the site 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.
Four gateway numbers are available for SMS — short codes for the United States, Canada, India, and an Isle of Man-based number for international use. There is also a short code for Vodafone users from the United Kingdom. Several third parties allow users to post and receive updates via e-mail.
Estimates of the number of daily users vary, because the company does not release the number of active accounts. In November 2008, Jeremiah Owyang of Forrester Research estimated that Twitter had 4-5 million users. A February 2009 Compete.com blog entry ranked Twitter as the third most used social network (Facebook being the largest, followed by MySpace), 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 growth of 1382%, Zimbio had a growth of 240%, followed by Facebook with a growth of 228%.