SAN FRANCISCO — Twitter has trained people to compress their thoughts into 140 characters and given a public stage to both dissidents in Iran and voluble stars like Shaquille O’Neal.

Now the startup appears to have chalked up another achievement. Twitter, which has no discernible revenue, is set to raise about $100 million of new funding that would value the company at around $1 billion, a person briefed on the company’s plans said Thursday.

For context, that is almost double the market capitalization of Domino’s Pizza, which has 10,500 employees and had $1.4 billion in sales last year.

Twitter has some 60 employees, and although it is experimenting with running advertisements on its Web site, Biz Stone, a Twitter founder, said this week at an industry conference that the company had no plans to begin widely running ads until 2010.

But Twitter’s cash infusion and exospheric valuation are not easily reduced to the level of the blind bets of past dot-com bubbles.

In its 3 1/2 years, Twitter has become a magnet for media attention, and its Web site now attracts 54 million visitors a month, according to comScore, the tracking firm. Along with Facebook, it is helping to remake the Web as a forum for the perpetual sharing of even the most trivial bits of information about people’s lives.

“There have probably been less than five examples of companies that have grown like Twitter has,” said John Borthwick, the chief executive of Betaworks, which created the link-shortening service Bit.ly. (Betaworks also invested in Summize, a Twitter search engine that Twitter acquired last year, and it now owns a small stake in the company.)

Borthwick lists Google, YouTube and Facebook as other examples. Twitter “represents a next layer of innovation on the Internet,” he said. “This investment is happening because it represents a shift.”

Is it worth it?

The new investors include Insight Venture Partners, a venture capital firm based in New York; T. Rowe Price, the mutual fund company, which is not normally known for placing such bets; and the current Twitter backers Spark Capital and Institutional Venture Partners.

The investment is likely to kick off more discussion about the heady valuations investors are assigning to some Internet startups, even as the U.S. economy struggles to emerge from a deep recession and the window for initial public offerings remains weak.

Twitter is not only what insiders charitably describe as “pre-revenue,” but the service has become known for going down periodically, although its reliability record has been improving lately.

To some, Twitter’s new valuation makes sense. Facebook, Google and Microsoft have all reportedly made entreaties to acquire the company, and its desirability to the Internet giants elevates its value.

Then there is the nonstop media attention, with everyone from Oprah Winfrey to local radio stations increasingly using the service to communicate with fans. “There is so much media hype around them, it was probably easy to go to mainstream investors and find someone who would be interested,” said Jeremiah Owyang, a social media analyst at the Altimeter Group.

‘Pulse of the planet’

Twitter has not yet commented on the investment, so it is not clear how it will use the new cash. The company does not appear to need the capital. It previously raised $55 million and has said it still has $25 million of that in the bank. But it is known to have wide aspirations to ultimately reach 1 billion users and become “the pulse of the planet,” according to internal documents that were illicitly obtained by a hacker and published on the blog TechCrunch earlier this year.

Twitter could use the investment to build the technology infrastructure required to grow to that scale. It also might use the cash to acquire one or more of the companies that are writing Twitter programs for mobile phones and computer desktops.

Twitter could even find a business model for itself if it were to buy one of the several startups devoted to helping companies manage their Twitter presence and monitor how their brands are being discussed.

But close followers of Twitter do not sense that the company is in any great rush to prove itself as a profitable venture.

“It would be trivially easy for them to turn on a revenue source today,” said Steve Broback, founder of the Parnassus Group, which runs conferences on Twitter and other business topics. “I don’t see that they are in a big hurry to start generating revenues, mostly because they want to minimize any sort of negative effect on their community.”