The rest, as they say, is history.
At the time, Dorsey and co-founders Biz Stone and Evan Williams said they had no idea just how big it would get, MediaBistro reported.
Since then, the microblogging site has come a long way.
According to the UK's Telegraph, Twitter logs about 500 million tweets per day and has an estimated 200 million active users. And that is a conservative estimate.
Twopchart estimates there are actually 500 million existing Twitter accounts.
To celebrate, Twitter released this fascinating video of its history, including its most popular tweet, and its influence on world politics.