Just as Facebook users thrill the world with their status updates, Twitter has evolved into a force of citizen journalism that's used not just to share the news, but also to break news.
From protests in Iran to Justin Bieber to the "Miracle on the Hudson," here are some memorable tweets.
April 10, 2008: James Buck tells followers: "Arrested"
James Buck (@jamesbuck), at the time a graduate student attending the University of California-Berkeley, was arrested while covering an anti-government protest in Egypt. After being apprehended, Buck tweeted "Arrested" to his 48 followers. One of his followers spread the word; less than 24 hours, Buck was free. He now has over 1,000 followers. (Read the tweet)
October 25, 2008: Army calls Twitter a "potential terrorist tool"
A U.S. Army intelligence report identified Twitter as a potential terrorist tool, citing the posting of "extremist ideologies and perspectives" as reason for caution.
November 5, 2008: Obama: "We just made history"
Recently-elected Barack Obama (@BarackObama) took to Twitter to thank his supporters and announce victory in the 2008 presidential election. Read Obama's post, "All of this happened because of you." (Read the tweet)
The president later admitted it wasn't he who posted that original tweet, but a staffer. His first real tweet came on the Red Cross' Twitter page. (Read the tweet)
PHOTOS: President Barack Obama
November 28, 2008: Tweeps report from Mumbai
Tweeps in Mumbai during the 2008 bombing and shooting attacks posted frequent updates and pictures that showed the drama unfolding over three days.
January 5, 2009: Justin Bieber's dead?
Faux celebrity death reports have been around as long as there have been celebrities. However, the tween uproar surrounding Bieber's alleged early-2009 death - after the pop star hadn't tweeted in 14 hours - was unique, and was the precursor to the current Bieber obsession on Twitter.
PHOTOS: Justin Bieber
January 15, 2009: 'Miracle' photo
Now known as the "Miracle on the Hudson," the national response to Captain "Sully" Sullenberger's emergency landing of US Airways Flight 1549 was kicked off by a photo posted to Twitter by Janis Krums (@jkrums). (See the original photo)
PHOTOS: Jet crashes in Hudson River
COMPLETE COVERAGE: Miracle on the Hudson
April 16, 2009: CNN and Ashton race to 1 million
At the time Nos. 1 and 2 in Twitter followings, Ashton Kutcher (in second entering the competition, @aplusk) challenged CNN (@cnnbrk) to see who could reach 1 million followers first. In conjunction with the contest, 11,000 mosquito bed nets would be donated in honor of World Malaria Day. Kutcher won the bet, but he no longer has the most followers on Twitter; that distinction belongs to Britney Spears (@britneyspears).
June 22, 2009: #Neda and Iranian protests
The amateur video taken of Neda Agha-Soltan dying in the street of Tehran was spread through Twitter, and helped feed a global response to anti-government protests in Iran. The graphic video's most iconic moment was a close-up of the dying woman's face.
The importance of Twitter to organizers and participants in the Iranian protests was underscored when the U.S. government asked the company to delay planned service maintenance that coincided with the turmoil, for fear of disrupting communication and coverage.
June 25, 2009: Death of Michael Jackson shuts down Twitter
When the King of Pop was rushed to the hospital and subsequently died, Twitter was forced to shut down due to the volume of tweets.
PHOTOS: Michael Jackson, year-by-year
January 13, 2010: Reports from the Haiti earthquake
Pictures from the scene of the quake, updates on family members, fundraisers such as Yele: while so many lines of communication with Haiti were down following the devastation, those with Twitter accounts took to the micro-blogging site to update the world.
SPECIAL REPORT: View our multimedia project, Loss and Hope amid the chaos in Haiti
MORE: Stories and photos of the Haiti quake and aftermath | How to help
March 5, 2010: Conan follows girl, girl becomes famous
Conan O'Brien (@ConanOBrien) was a little late giving into the Twitter wave, with his first post on Feb. 24, 2010. Additionally, he avoided following others, until announcing on March 5 that he would follow one person. That person, Sarah Killen (@LovelyButton), would gain social media notoriety, and has continued to enjoy the benefits of being Conan's lone Twitter bud.
MORE: Read about Conan's new 'friend'
PHOTOS: Conan O'Brien
April 14, 2010: Library of Congress indexes all tweets
Who knew Congress cared what time we were going to dinner, or how hung over we were? Aiming to embrace new media, every tweet ever sent -- and every tweet yet to be sent -- is filed in the historic library. Oh-oh...
May 30, 2010: 15 billion tweets
Twitter hit 15 billion tweets sent - or 2.2 tweets-per-person on Earth - in late May. The landmark came just three months after the 10-billion-tweet landmark.
Photo of Barack Obama by Getty Images
Photo of US Airways Flight 1549 by AP
Photo of Michael Jackson by AP
Photo of Conan O'Brien by AP