Twitter is the platform most associated with the origin of hashtags. Chris Messina proposed using the # symbol to group related posts on Twitter in 2007, turning an older typographic character into a social-media navigation tool.
The idea had an earlier technological precedent. Internet Relay Chat, or IRC, used the hash symbol to identify channels and discussion topics from around 1988. Messina adapted that convention for Twitter, where users could place a word or phrase after # to create a searchable label. This allowed communities to organize conversations without waiting for Twitter to create a formal topic system.
Twitter did not initially promote the idea strongly, but hashtags gained visibility during the 2007 San Diego County wildfires. The tag #sandiegofire helped users find updates about the event and showed how the system could be useful in real-time situations.
Hashtags are now common on Instagram, Facebook, YouTube, TikTok, and other services. Those platforms popularized new styles of hashtag use, but Twitter remains the platform historically linked to their social-media origin and early growth.