Twitter is a 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 bytes in length. Updates are displayed on the user's profile page and delivered to other users who have signed up to receive them. Senders can restrict delivery to those in their circle of friends (delivery to everyone being the default). Users can send and receive updates via the Twitter website, SMS, RSS (receive only), or through applications such as Tweetie, Twitterrific, Twitterfon, TweetDeck and Feedalizr. The service is free to use over the web, but using SMS may incur phone services provider fees.
As of March 2009, Twitter has received extensive visibility and popularity worldwide. Twitter is often described as the 'SMS of Internet' in that the site provides the back-end functionality (via its APIs) to other desktop and web-based applications to send and receive short text messages often obscuring the actual website itself. This extensibility of the service has earned it more popularity than it would have gained if users had to visit the site to use the service.
Four gateway numbers are currently available for SMS: short codes for the United States, Canada, and India, and a United Kingdom-based number for international use. Several third parties offer posting and receiving updates via email.
Estimates of the number of daily users vary as 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 ranks Twitter as the third largest social network (MySpace would be second and Facebook would be the largest in the world), and puts the number of unique monthly visitors at roughly 6 million and the number of monthly visits at 55 million.