This is the year a 5 1/2-year-old changed the world.

The name of this mighty mite? Twitter.

From the Arab Spring to the Japan earthquake and tsunami, from the death of Osama bin Laden to the Occupy movement, Twitter played a big role, with millions learning, reacting, connecting, and sharing instantly - far faster than old slowpoke media like radio and TV.

"Twitter? Oh, it's definitely mainstream now," says Kate Bussmann, author of A Twitter Year: 365 Days in 140 Characters.

"I see 2011 as the year of democratization for Twitter," says William L. Weaver, an associate professor of integrated science, business, and technology at La Salle University. "It's in the hands of Everyman, and Everyman is using it."

Born July 2006, Twitter now stands at 100 million users strong, and they tweet 230 million times a day.

"This is a dramatic coming-out of Twitter as a bullhorn for public events, a means of social organization," says Lee Rainie, director of Pew Research Center's Internet & American Life Project. "For many people, it's now the default, the reflexive move."

Proof people care

The year's hottest Twitter topic - seen in hashtags, that little pound sign (#) in front of a word or phrase that lets people find tweets containing it - was #egypt. No. 2 was #tigerblood, courtesy of the No. 1 most-tweeted-about actor, Charlie Sheen. (Liz Taylor was the top tweeted-about actress.) The top five most-tweeted-about places: Cairo, Egypt, Japan, Libya, and Tokyo.

You can track what people care about through the rates of tweets per second (TPS) worldwide at any given time, and also through the "trending topics" - topics of especially rapid growth in tweets.

The world record for TPS - 8,868 - had been about the baby bump for Beyoncé and her pregnancy announcement Aug. 28 at the MTV Music Video Awards. But on Dec. 9, that was smashed to bits by a furious eruption in Japan during the showing of the anime film Castle in the Sky, with a mind-numbing 25,088 TPS.

Sports and entertainment, up among earthquakes and antiterrorist raids? That shows Twitter is the province of the young, but it also shows that millions of us are multimedia mavens.

As Bussmann points out, "Many of these tweets are people sharing what they just saw on TV, or on the Web - they're using two screens, and two media, at once."

Rainie says, "What leaps out at you is the participatory part. We're no longer passive observers. We're leaning forward, commenting about the news, sharing it with others, and that's a new, different order of engagement with news and popular culture. We're adding to culture, producing it, not just looking at it."

The story of 2011, Rainie says, is how all these media are blending: "People tweet about YouTube, send pictures on TwitPics, post on Facebook about Tumblr. These once-new media are converging, thanks to the users."

Emergence, just in time

Rainie is on to something. This year, millions of people cooked up new uses for Twitter and other social media, often in undreamed-of combinations. "Emergence is a fantastic term for what we're seeing," says Weaver. Emergence, glittering term, refers to the way new, large, complex structures arise out of millions of smaller, simpler interactions - how, for example, a social-protest movement can arise out of a million tweets.

It's not just myriad business folks and celebs tweeting ads and updates. Individuals and small groups showed creativity, too. Ailing musician Chris Strouth tweeted, "I need a kidney," and got one. Kirti Dwivedi used Facebook to search for a kidney for her mother, Anu, and a donor contacted her via Twitter. (There are now dozens of organ-donation accounts on Twitter.) A Japanese fishing association figured out a Twitter way for fishermen to sell their catch before returning to port.

Via Facebook and Twitter, Wael Ghonim, a Google exec, helped rally support for political change in Egypt. When he went there, he was jailed secretly for 11 days. Notice?: Ghonim is still around, but Hosni Mubarak's government isn't. Ghonim's memorable conclusion: "If you want to liberate a government, give them the Internet."

The Arab Spring was a prime example of people finding new uses for Twitter, even in countries where not many people use social media. "Facebook was used to organize protests and events," says Bussmann, "and Twitter was used to spread the word, give updates, warn protesters about police activities, and call for backup. Protesters also used Twitter to tell the outside world what was going on." Western governments were slow on the uptake, but the trending topics on Twitter showed that people all over the world supported the Arab Spring.

Twitter was crucial during Japan's earthquake and tsunami. In many parts of Japan, phone lines were down, but texting and Twitter were available, enabling victims to contact loved ones and relief services. The Salvation Army, the Red Cross, and other agencies set up Twitter accounts to coordinate donations, volunteer efforts, and supplies, as well as to give updates. "This happened in Haiti, too," Bussmann says. "As of 2011, it's pretty much what happens in disasters."

She calls the London riots of Aug. 6 through 12 a "tipping point." Dan Thompson, Sophie Collard, and Sam Duckworth organized a leaderless effort via Twitter to clean London streets after the riots. So many people showed up all but instantly that, in Bussmann's words, "there wasn't enough work."

There's a principle in business called "just-in-time," or JIT, in which companies produce inventory according to demand, on the fly. JIT keeps you efficient, agile, responsive. In the same way, Twitter and social media let individuals and institutions leap into action almost immediately in time of crisis. Weaver calls it "just-in-time philanthropy." Nice!

Occupy might be the ultimate example of Twitter social organization. "Occupy self-styled themselves as social-media operators," says Rainey. "They use Facebook, Twitter, Foursquare, and YouTube to get the word out and give a sense of cohesion."

Khadijah White, a grad student at the University of Pennsylvania's Annenberg School for Communication and also an Occupy facilitator, says, "It was savvy and crucial how Occupy coordinated their Facebook pages with Twitter, both locally and worldwide. At a very complex time, it kept people informed and cohesive."

In October, at the height of Occupy Philadelphia, Chris Goldstein, social-media coordinator, was giving updates, coordinating events, and directing donations via Twitter. "Social media is how the camp eats, sleeps, and keeps it together," he said. "Thanks to Twitter, this is one Philadelphia neighborhood that never goes hungry."

Why Twitter's sexy

The key to Twitter's allure, according to Bussmann, is that "it turns what used to be a solitary experience into a mass social experience."

But are such relationships real? "I wouldn't be surprised if, when the phone was first invented, they were asking the same question," says Bussmann. Twitter, she says, is "just one more way of reaching out."

2011 Hot Topics: Tweets Per Second

New Year's (Jan. 1): 6,939

Super Bowl XLV (Feb. 6): 4,064

Japan's earthquake/tsunami (March 11): 5,530

Royal wedding of Prince William and Kate Middleton (Apr. 29): 3,966

Killing of Osama Bin Laden (May 2): 5,106

UEFA Champions League soccer (May 28): 6,303

NBA finals (June 13): 5,531

BET Awards (June 27): 6,436

MLB All-Star Game Home Run Derby (July 11): 4,995

Brazil eliminated from Copa America (July 17): 7,166

East Coast earthquake (Aug. 23) 5,449

Japan beats United States, Women's World Cup (July 17): 7,196

Beyoncé pregnant, announced at MTV music awards (Aug. 28): 8,868

Steve Jobs resigns as Apple's CEO (Aug. 25): 7,064

Troy Davis executed (Sept. 20): 7,671

Steve Jobs dies (Oct. 6): 6,049

Japanese anime TV show Castle in the Sky (Dec. 9): 25,088

Source: Dates listed are of Twitter traffic, not the date of the event. Twitter's 2011 Year in Review