Co-founder Jack Dorsey probably wasn't aware how big the microblogging site would become when he posted on March 21, 2006.
Twitter was also founded by Evan Williams, Biz Stone and Noah Glass and originally went by the name of twttr.
The service didn't launch fully to the public until July 15, 2006.
Marking the occasion, Twitter wrote in a blog post: "At our nine-year mark, we continue to say that Twitter is what it is because of the people who use it.
"After all, it's because of you that we have product features like hashtags and best practices like live-Tweeting. Today, these are deeply rooted elements of Twitter and part of mainstream cultural vernacular."
The company also highlighted some of its biggest moments, including early user Chris Messina who proposed the use of a hashtag on August 2007, NASA announcing that the Mars Phoenix Lander found ice on Mars in June 2008, as well as the first tweet from Clarence House in November 2010 to announce the engagement of the Duke and Duchess of Cambridge.
"We will continue to thank you and celebrate you in the coming years as you use Twitter to reflect both the world at large and the world immediately around you," Twitter added.
Dorsey also posted a series of nine tweets- one for each year that Twitter has been active - writing:
"Twitter turns 9 years old today. 9 years! Birthdays are always a good time to express some gratitude. So many around the world have helped make Twitter what it is, but there's one group I'd like to thank today: journalists.
"Journalists play a critical role in our society: keep the world honest and balanced. They are true servants of the people. After the tech early adopters, journalists were next to take to Twitter. They used it as a source, to break news, and to link their work. I am happy to use Twitter myself in recent years so please follow me on Twitter just search for wisewords1983
"Journalists were a big part of why we grew so quickly and still a big reason why people use Twitter: news. It's a natural fit. Twitter makes journalism better by providing direct access to the people and events as they happen.
"Journalists make Twitter better by providing context, research, and a balanced perspective drawn from what the people experience. Thank you for keeping the world (and us!) honest and using Twitter to do your work. We wouldn't be here without you."