Twitter is celebrating its official fifth birthday today, which marks the first time a tweet was sent on the site.

The first-ever tweet was sent by co-founder Jack Dorsey, simply stating: "Inviting coworkers".

Dorsey has spent the last week reminiscing about the beginning of the micro-blogging service, posting the instant message conversation he had with Biz Stone where he announced twttr (it's orignal name) was ready and the website's first logo – which was a purple blob, with the strap, 'an Odeo thingy'.

Twitter's rise has been undeniably impressive, with the site now hitting a billion tweets a week and employing over 400 people.

Celebrity endorsement

Although four out of the top five most-popular people on Twitter are celebrities (Lady Gaga is number one, followed by Justin Bieber, Britney Spears, Barack Obama and Kim Kardashian), Twitter is now being recognised as a place where news breaks first, from the recent events in Japan to the death of Michael Jackson.

Not that this has convinced the likes of Robert De Niro to get on board, who recently said in an interview with Parade magazine: "I use the computer, but Twitter's not something that I'm into. I think it's great, and it's not that I don't understand it, but in a way, I don't get it and that's okay."

It seems that many are getting Twitter, however, with 460,000 new accounts created on Twitter every day and the site is currently valued at $4 billion.

A price that must put a smile on all the co-workers' faces that were invited by Dorsey five short years ago.