On March 21, 2007 "Twitter cofounder Jack Dorsey blasted off the very first tweet. What began as an experiment in "microblogging" -no more than 140 characters, please- has become a cultural landmark :
Twitter now has 200 million users, including tech luminaries, celebrities and the president of the United States. It started off slowly. In 2007, Twitter averaged just 5,000 tweets per day. But in 2009, the site hit a tipping point and became a broadcast channel for major news events.[...]"
Full article on CNN.money.com