(Kimihiro Hoshino/AFP)

Sent by Twitter co-creator Jack Dorsey, it ushered in a program that has revolutionized the way news is delivered and credited for spurring on social unrest in the world. Will that tweet go down in history along with Alexander Graham Bell’sMr. Watson, come here. I want to see you”?

just setting up my twttrless than a minute ago via webJack Dorsey

Twitter became something its founders Biz Stone, Evna Williams and Dorsey never imagined. It became a news device, an alert system, a rallying point and a megaphone for celebrities. It has heralded social change and it has introduced a whole set of annoying cute words to our lexicon (put “tw” in front of any word...). In honor of its fifth anniversary, the site launched Discover Twitter, a new site that helps navigate the tworld. (See it works every time!)

Here’s a look back at some of the most memorable tweets:

The tweet that Biz Stone said showed him the power of the tool he had created happens to have been sent out by the Post’s own James Buck. In 2008, then a graduate student, Buck sent out this message to his followers after being arrested in Egypt. When his friends saw it, they contacted the consulate and helped get him released:

During the 2009 green revolution, one user Persiankiwi became a source of constant information about what was happening inside Iran. Using the service to rally support outside the country and to organize protests within, Persiankiwi became a model for the revolutionaries across the Middle East to follow:

Tonight streets of north Tehran - large numbers of ppl gathering - Sea of Green - #Iranelectionless than a minute ago via webpersiankiwi

The mocking comedic genius of BP Global PR’s parody account kept the heat on the oil giant for months after the Gulf spill. While not the first parody account, it was by far the most well known and showed how a little humor can become a major PR disaster for a brand.

We regretfully admit that something has happened off of the Gulf Coast. More to come.less than a minute ago via webBP Public Relations

Twitter user Janis Krum sent out the first image of the plane that landed in the Hudson River in 2009. It signalled the advent of a new kind of news gathering and dissemanation.

http://twitpic.com/135xa - There’s a plane in the Hudson. I’m on the ferry going to pick up the people. Crazy.less than a minute ago via TwitPicJanis Krums

Your first tweet. If you have under 3,000 tweets you can find your tweet here. The reason why this is so important? All together, we’re producing a healthy 1 billion tweets a week, or 140 million tweets a day. Not too shabby for a five-year-old.

Just set up my professional twitter profile for @seattletimes. Can’t believe I’m handing over my anonymity so easily.less than a minute ago via webT.J. Ortenzi

Let us know what your first tweet was, or tell us why Twitter matters to you by using #HappyBirthdayTwitter.