Twitter 5th Birthday: Lady Gaga, Justin Bieber, Barack Obama Most Followed

July 15, 2011 — -- It's been five years since Twitter co-founder Jack Dorsey sent out the first experimental tweet: "Inviting coworkers."

That's all it said.

Today marks the five-year anniversary since Twitter.com became a public website, and like every Tweet since, Dorsey's was limited to 140 characters.

Since then, celebrities from Lady Gaga to Charlie Sheen to President Obama (or the staffers tweeting on his behalf) have mastered the highly truncated form of language that Twitter has made popular; others (think of Rep. Anthony Weiner) have been less adroit.

How do you mark the day? With a so-called hashtag: "#happybirthdaytwitter."

Click Here for Pictures: Celebrities on Twitter

Twitter is now the 9th-most-visited site worldwide, according to the Web-tracking service Alexa. Google, Facebook, YouTube and Yahoo may get more users, but they can't top Twitter for the sheer number of messages sent.

As of last week, Twitter said users were sending 200 million tweets per day, up from 2 million in January 2009. If you printed that on paper, at a rate of 20 tweets per page, it would fill the equivalent of 8,163 copies of Leo Tolstoy's "War and Peace," the company said.

Despite the flighty name and the forced brevity of the messages, Twitter has become a major, and often serious, medium. From the earthquake and tsunami in Japan to the uprising in Egypt, it has been a way for people to connect, share links to pictures and news stories, or watch what is collectively on the world's mind.

And of course, it's become the gathering place for the rabid followers of pop culture icons.

Ashton Kutcher won the race to become the first Twitter member to 1 million followers, but he's since been passed by a few pop stars, a reality TV queen, and one commander in chief.

ABC News did a search, and as of this morning, the most-followed figures on Twitter are:

The site has also become a barometer of what's hot and what's not. Twitter provided its own list of what's been trending the first half of 2011 in popular culture:

And as with any unfiltered forum where citizens speak their minds, there have been hordes of memorable tweets along the way. Here's our list, as well, of some of the more celebrated: