Social networking and microblogging service utilising instant messaging, SMS or a web interface. [Description from dmoz]
Q: What are you doing? A: I'm reading Hoover's profile on Twitter. The company operates a free Web site that blends social networking with the ability to post short messages, or micro-blogs, commonly known by users as "tweets." Technically, in order to "tweet," Twitter wants users to answer one question: What are you doing? The response, which is limited to 140 characters or less, is posted to a personal Web page that can be restricted to select viewers, or viewed by anyone. Unlike instant messaging, viewers do not reply to Twitter postings. Tweets are designed to be accessed through the Internet and on mobile devices. CEO Evan Williams, Jack Dorsey, and Biz Stone co-founded Twitter in 2006. [Description from Hoover's]
This site reaches over 95 million U.S. monthly people.The typical visitor buys from Microsoft, uses eHarmony, and frequents Senior People Meet.