Hashtag

Chris Messina proposed it on Twitter in August 2007, reasoning that IRC had used # for channels for twenty years already. Twitter dismissed the idea as too technical for ordinary users, but it spread among users anyway, and when that autumn's San Diego wildfires gathered under #sandiegofire the value became obvious. Twitter started linking tags automatically in 2009.

The feature has since been demoted almost everywhere. Algorithmic feeds made the tag redundant for discovery, spammers rendered popular tags unreadable, and Instagram announced in 2024 that it was dropping support for following tags. What remains is the cultural use — the tag as ironic aside or campaign name, with #metoo the clearest case of a search string becoming a movement.