NEW YORK — The pope. President Barack Obama. Queen Elizabeth. Oprah. You.

When Twitter started seven years ago as an obscure medium for geeks, critics dismissed it as an exercise in narcissism. Some thought it would be as intriguing as watching people gaze at their bellybuttons. But it quickly matured into a worldwide messaging service used by everyone from heads of state to revolutionaries and companies trying to hawk products.

Now, Twitter is taking the next critical step in its evolution: selling stock to the public. It promises to be the most hyped and scrutinized initial public offerings since Facebook’s Wall Street debut in May 2012. To be successful, the company will need to become an advertising behemoth and prove that the same service that has helped change the course of history also can make money.

Twitter quietly slipped out news of its plan to go public in a tweet Sept. 12. By the next morning, nearly 14,000 of Twitter’s 200 million users had retransmitted the message.

“Twitter epitomizes the revolution of social media ... more than Facebook, more than YouTube,” says Fordham University communications professor Paul Levinson, author of “New New Media.” “It caters to the immediacy, the equality of all users.”

And yet, Twitter really isn’t that big. Only about 15 percent of Americans say they’ve ever used it, according to an August poll by the Pew Internet and American Life Project. At the time of Facebook’s IPO, an AP-CNBC poll found that 56 percent of Americans said they had pages on Facebook. Twitter’s 200 million global users represent about one-sixth of Facebook’s 1.16 billion.

Even so, Twitter generates more news than Facebook. A big part of that is its public nature, Levinson says. With their messages of 140 characters or less, most people tweet openly, allowing the world a glimpse at their thoughts. Facebook, in contrast, gives its users a plethora of controls to hide or show posts to as many or as few people as they’d like. That means many users share updates only with people they already know.

“You can rub elbows with famous people instantly,” Levinson says, noting that people can send a message to a movie star just as easily as they communicate with a friend. “That’s what makes communication in the 21st century radically different from any time in the past. It wasn’t until Twitter that the combination of speed and access to anyone became available for everyone.”

Twitter might never have become the world’s digital watercooler if Noah Glass and Evan Williams had persuaded more people to tune into a podcasting service called Odeo started in 2005. Less than a year after its birth, Odeo was destined to be a dud.

By early 2006, Glass and fellow Odeo programmer Jack Dorsey began to work with co-worker Christopher “Biz” Stone on a way to corral the menagerie of text messages typically sent by phone. It was Glass who came up with the original name Twttr in a reference to chirping birds. On March 21, 2006, Dorsey posted the first tweet: “Just setting up my twttr,” and Glass posted the same 10 minutes later.

By 2007, Twitter was incorporated with Dorsey as CEO and Williams as chairman. Dorsey and Williams would eventually swap roles. Both remain major shareholders. Dick Costolo, a former Google executive and once an aspiring stand-up comedian, is now CEO.

Perhaps Twitter’s greatest appeal is that it allows users to see news unfold in real time. And they can witness history, such as 2011’s Arab Spring protests.

Like Facebook, Twitter reaps most of its revenue from advertising. Research firm eMarketer expects Twitter’s annual ad revenue to hit $1.33 billion by 2015.