The hashtag on Wednesday celebrated 10 years of making social media just a bit more easier to navigate.
The sign has been used to highlight keywords online around the world since 2007, when it got its first outing on Twitter.
Before long, it had spread to other social media platforms, including Facebook, Instagram and Tumblr.
It was Chris Messina, an American designer and social media expert, who originally proposed using the hash sign (#) to group tweets by subject.
He made the original suggestion in a tweet posted on August 23, 2007, before explaining more in a post online a couple of days later.
He launched the first-ever hashtag, #barcamp, to identify a set of conferences focused around technology and the web that he helped organise.
Messina, who describes himself as an “avid Twitterer”, has sent more than 39,500 tweets in 11 years.
Wednesday’s birthday celebrations, of course, trended with #Hashtag10.
Today 125 million hashtags are exchanged every day, often serving as a point to launch massive online campaigns.
In April 2014, the abduction at Chibok, in northeastern Nigeria, of 276 schoolgirls by Islamists from Boko Haram led to the posting of the #BringBackOurGirls hashtag.
The then-US first lady Michelle Obama was among those who used it to draw attention to the fight against Boko Haram.
#BlackLivesMatter was another digital rallying cry, going viral during a wave of protests over the deaths of several black people at the hands of the police in the United States.
Then there was #OccupyWallStreet for the American movement that set up a protest camp in the heart of New York’s financial district to protest against financial greed and corruption.
Hashtags have also sprung up in the wake of terror attacks to allow internet users to express support for the victims and survivors.
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In 2015, #JeSuisCharlie was shared five million times in the two days after the January 7 jihadist attack on French magazine Charlie Hebdo in Paris, which killed 12 people.
#PrayforParis was tweeted more than six million times after the November 2015 attacks in and around Paris which cost 130 lives.
The slogan was adapted elsewhere, for example in Berlin – #PrayforBerlin – after a truck attack left 12 dead at a Christmas market in the German capital, in December 2016.
But the hashtag has also been used on a lighter note, to follow stunts or running jokes.
Videos of the #IceBucketChallenge, a challenge to dump a bucket of ice and water over a person’s head, helped raise US$100 million to fund the fight against motor neurone disease.
Funny hashtags have proved extremely successful. The mistakes of politicians are often mercilessly tagged and mocked online.
Word games, photos and gifs add to the mix on topics as varied as the weather, celebrities, and football matches.
A look at the top three hashtags on Twitter last year gives an idea of the kind of subjects capable of going viral.
In the gold medal position was #Rio2016, for coverage of the Rio Olympics in Brazil. Fans, as well as athletes, made use of the hashtag.
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Political hashtags can also take off. #Election2016, for coverage of a controversial US presidential election campaign, was the second most popular hashtag.
But entertainment-related items can also score highly. #PokemonGo went viral during the launch of a new mobile phone version of the Nintendo game, becoming the third most popular hashtag last year.
Other hashtags are more multi-purpose ones. #RIP had a good run last year, after a slew of celebrity deaths made it the ninth most popular hashtag on Twitter.