Millions of new users may be flocking to Twitter, but most are flying the coop within a month, a market research firm has found.

That will ultimately limit the audience of the social networking site to a small fraction of internet users, despite its recent explosive growth, says Nielsen Online.

"There simply aren't enough new users to make up for defecting ones after a certain point," wrote Dave Martin, the firm's vice-president of primary research in the company's blog Tuesday.

Nielsen Online reported that Twitter's audience more than doubled in March. In February, the site had more than seven million unique visitors — almost a 14-fold increase from 475,000 the year before.

And the site's profile recently got a hearty boost from celebrities such as talk show host Oprah Winfrey and actor Ashton Kutcher.

However, currently 60 per cent of Twitter users don't come back the next month, Martin wrote — and that's already an improvement.

"For most of the past 12 months, pre-Oprah, Twitter has languished below 30 per cent retention," he said.

Martin provided a chart to show that with its current retention rate, Twitter ultimately can expect to reach a maximum of just 10 per cent of internet users.

MySpace, Facebook, keep 70 per cent

In contrast, MySpace and Facebook, two other popular social networking sites, kept about twice as much of their audience coming back when they were at a stage similar to where Twitter is.

"When they went through their explosive growth phases, that retention only went up, and both sit at nearly 70 per cent today," Martin wrote.

He suggested that Twitter needs to establish a higher level of user loyalty if it expects to keep up its growth.

Twitter is a social networking site where users can post updates or "tweets" of up to 140 characters from the web or a mobile device such as a cellphone. The tweets are automatically sent to people who have signed up to "follow" them.

In the past two months, the site has basked in celebrity publicity.

On April 17, actor Ashton Kutcher announced he had won a high-profile race with U.S. news network CNN to gain 1 million followers on Twitter. The same day, talk show host Oprah Winfrey hosted a Twitter special with Kutcher as guest and sent her first tweet.