SAN FRANCISCO: More than 60% of American web users who sign up to Twitter, the online microblogging service, don't visit the site over the following month, giving it an audience retention rate of just 40%, Nielsen Online has found.

The growth of Twitter has been one of the major features of the digital landscape in recent months, and eMarketer has estimated that 18.1 million US web users will be active on the site by 2010.

Nielsen has assessed the behaviour of new members of the social media portal, and found that recent publicity regarding the service has helped lift its audience retention rate from the previous level of 30%.

However, it also estimates that the current retention rate of 40% would limit Twitter's online penetration to around 10% of Americans with web access, with a retention rate of around 70% being required to lift a website's total online reach to 50%.

David Martin, vice president of primary research at Nielsen Online, argues the site is still a "fledgling", and agrees it has grown "exponentially" over just a few months, helped by media and celebrity support.

However, he also suggests it is not performing as well in keeping its audience as Facebook or MySpace did when they were still emerging networks.

Facebook's audience retention rate has only fallen below 50% once, and has remained steady at around 60% during the site's short lifespan.

MySpace started with a retention rate of around 30%, a figure that was under 20% for Twitter at the same point in its expansion.

However, MySpace has enjoyed steady growth in audience retention, which now stands at a similar level to Facebook, with both sites having a much higher number of users than Twitter.

As such, Martin argues that "Twitter has enjoyed a nice ride over the last few months, but it will not be able to sustain its meteoric rise without establishing a higher level of user loyalty."

Data sourced from Nielsen Online; additional content by WARC staff