Twitter did not come up with the idea on its own. It was actually a user of the network, Chris Messina, who on August 23, 2007, suggested regrouping posts on the same theme with a keyword preceded by an “#”. The idea was rapidly adopted by other members of the network, and thereafter embraced by other social media services, to the point where hashtags are now omnipresent.
Over the years, numerous hashtags have gained in popularity to the point where their widespread use has become viral. Examples include #BlackLivesMatter, #IceBucketChallenge and #JeSuisCharlie. A marketing instrument for brands, hashtags are also used to defend causes, as was recently the case with #MeToo.
According to experts at Twitter, a good hashtag should be easy to read and write, and easily memorized. As a rule, it is better not to spell out hashtags in capitals, and users are encouraged not to use too many (no more than two per tweet).
On Twitter, the anniversary is referenced by the hashtag #HashtagDay.
how do you feel about using # (pound) for groups. As in #barcamp [msg]?
— Chris Messina is at )'( (@chrismessina) August 23, 2007