On what date was Twitter publicly launched?

Twitter was publicly launched on July 15, 2006.

The date marks the public introduction of the service's full version, after an earlier prototype had been used internally by employees of Odeo. Twitter's origins trace to a 2006 brainstorming session at Odeo, where Jack Dorsey proposed using SMS messages to share short status updates with a small group. The project was initially styled “twttr,” partly echoing the short names used by SMS services.

The public launch did not immediately make Twitter a global sensation. The company continued refining the product, and its audience grew substantially after Twitter received attention at the South by Southwest Interactive conference in March 2007. Twitter later spun off from Odeo and became its own company in April 2007.

A frequent mix-up is confusing the public launch with the first post, which Dorsey sent on March 21, 2006, or with Twitter's later corporate milestones. July 15 refers specifically to the service becoming publicly available, not the date of its founding or first message.