Twitter is a website which offers a social networking and microblogging service, enabling its users to send and read 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 just their followers.
Twitter was created in 2006. Since then Twitter has gained popularity worldwide and is estimated to have 200 million users, generating 65 million tweets a day (2010)
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Twitter from Wikipedia