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

PHOTO: Celebrities from Lady Gaga to Charlie Sheen have mastered the highly truncated form of language that Twitter has made popular.PlayABC News
WATCH Twitter Turns 5: Who Is the Most Followed?

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 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:

1. Lady Gaga -- 11,644,682 followers

2. Justin Bieber -- 11,037,025

3. Barack Obama -- 9, 121,529

4. Britney Spears -- 8,530,677

5. Kim Kardashian -- 8,306,783

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:

1. Rebecca Black, the pop singer

2. Femme Fatale (Britney Spears' new album)

3. Charlie Sheen

4. #tigerblood (a hashtag popularized by Charlie Sheen)

5. Nate Dogg, the rapper

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:

Lady Gaga, on the Senate's vote to let gays serve openly in the military: "Can't hold back the tears+pride. We did it!"

Kirstie Alley on her weight: "I've lost over 50 lbs...and I'm having the time of my of my life...30 more to goooooooooooooooooooooooooooooooooo."

Sen. John McCain (R-Arizona), who survived as a POW in Vietnam and overcame skin cancer before becoming the Republican nominee for president in 2008, shared a laugh after Snooki Polizzi, of "Jersey Shore" fame, complained she'd give up tanning beds because President Obama would tax them. "@Sn00ki u r right, I would never tax your tanning bed!" said McCain. "Pres Obama's tax/spend policy is quite The Situation. but I do rec wearing sunscreen!"

@Twitter: #Happybirthday! 5 Yrs Old!

Twitter's San Francisco staff has not done much publicly to mark today; they had more of a celebration in March, the five-year mark from when they put the site online for so-called beta testing. That kind of thing is common with tech startups; Google says it celebrates its anniversary on different days in September, "depending on when people feel like having cake."

What was on people's minds today? It's a summer Friday, and some of the topics Twitter listed as trending showed a pattern: Neville Longbottom ... Alan Rickman ... Gryffindor -- you get the idea. People were commenting on the new Harry Potter film.

So there we are, tweeting on the serious and the salacious, the weighty and the light-hearted, the trivial and the truly important. It's where Jim Carrey, the actor, posted a picture of his new grandson: "This is what 7lbs 11 oz. of Calfornia [sic] dynomite [sic] looks like!"

ABC News' Brian Canova contributed to this story.