Twitter's influence is still dwarfed by Facebook, which has more than half a billion users and a vast global reach. A recent Pew Internet & American Life Project study showed that only 8% of adult Internet users in the U.S. use Twitter.
"In some ways people have found that Twitter is not the be all, end all Democratic platform," Internet culture expert Tim Hwang said.
Yet Twitter is one of the rare Internet companies to make such a big splash in the mainstream. It has been likened to social networking service Facebook and Google in their early days. Twitter has become so ubiquitous that, like Google, tweet is now a verb.
The success of Twitter has turned its low-key founders Stone and Williams into Internet celebrities. Twitter's San Francisco headquarters has played host to Russian President Dmitry Medvedev, who sent his first tweet from there, and hip-hop star Kanye West, who performed lyrics from his upcoming album.
And Stone was recently a surprise guest on "The Colbert Report" to give host Stephen Colbert the first-ever Golden Tweet award for the most retweeted (as in popular) update of 2010 about the BP oil spill: "In honor of oil-soaked birds, 'tweets' are now 'gurgles.'"
Despite all the hobnobbing, Twitter's offices are as democratic as the service. Company executives sit elbow to elbow at long desks and lunch at small tables with employees. Staffers flock to meeting rooms named for different types of birds. They take breaks to learn "twideokinetics," a combination of meditation and martial arts, with Twitter Chief Technology Officer Greg Pass or take a jog with Tweetfeet, the company running club. Each new employee gets a copy of Twitter's 10 operating principles ("be a force for good" and "make things happen" among them).
Stone said he was proud of Twitter's corporate culture, which has drawn top recruits from Facebook, Google and YouTube.
"Twitter was built on the fundamental belief that people are good and that if you give them the tools, they will do good things," Stone said.
Now Costolo is bringing business discipline to Twitter. The 350-person company is building data centers to cope with crushing traffic and has hired 250 people in the last year alone. It's trying to combat its image as an innovation laggard. And it has clarified its vision: It's billing itself as an information network that connects people to what's important to them as it happens.
Costolo is the quintessential operating executive whose job is to bring "order, focus, calm and execution," Twitter investor Wilson said.
For much of its young life, Twitter had no real plan except to keep up with its explosive growth — and to keep its users from seeing the "fail whale," the cuddly cartoon mammal that surfaces when Twitter is flooded with too much traffic.
Co-founder Williams said he was amazed that the service kept growing in popularity even when for so long there were so many hurdles to using it. Last fall he oversaw a redesign of Twitter's website that made it easier for users to post photos and videos, and easier for marketers to reach out to those users.
"Now we have gotten to the point that we can rise to the challenge," Williams said. "We are building on the lessons of the last couple of years that have shown us what Twitter is and how to run it."
The challenge is twofold: making the product easier and more engaging and, most important, making money.
Social media networks have been reluctant to flood their sites with ads targeting the millions of people who interact with one another online. Instead they have sought out less obtrusive and more natural ways to connect users with marketers.
For Twitter, that has meant trying to come up with ads that are so relevant and useful that users don't think of them as ads.
Costolo claims Twitter has "cracked the code" on that kind of advertising and predicts advertisers will soon spend millions of dollars on Twitter. Analysts say Twitter's efforts so far are promising.
Called Promoted Tweets and Trends, these ads show up at the top of Twitter feeds or trends. Advertisers pay anytime someone interacts with the Twitter update by clicking on a link, forwarding the update to friends or replying to it.
On average 5% of Twitter users who see a promoted Tweet interact with it, a rate that Costolo said was "an order of magnitude greater" than most online ad campaigns. Promoted Trends go for as much as $100,000 for a single 24-hour campaign.
Virgin America, for one, is a fan of Twitter advertising. The airline promoted the launch of its new service to Toronto in April by offering special discounts to Twitter users. The promotion gave Virgin America its fifth-highest sales day.
"We have definitely seen great success in partnering with Twitter," said Jill Fletcher, Virgin America's social media and communications manager.
But some advertisers are not yet sold. Costolo is banking that will soon change. He concedes that there is "a ton of work left to do on the advertising platform," but says he has never seen an advertising business work so well so quickly.
Twitter is ramping up by building a national sales force, hiring executives from Facebook and Google, and expanding its reach overseas, where it is experiencing torrid growth. It must continue that pace to succeed as a major Internet player.
"We all believe that Twitter could be one of the great Internet companies," Costolo said. "We know we have to prove that, and we know we have a lot of work to do."