Twitter may be enjoying a huge surge in popularity, but it has a hard time keeping hold of its users, according to new data from Nielsen Online.

The research firm said that Twitter logged seven million hits in February, compared to under 500,000 in February 2008. However, over 60 per cent of users abandon the service after a month or less.

"Twitter's audience retention rate, or the percentage of a given month's users who come back the following month, is currently about 40 per cent," said David Martin, vice president of primary research at Nielsen Online, in a blog post.

"For most of the past 12 months, Twitter has languished below 30 per cent retention."

The high drop out rate will be damaging to Twitter, since these retention levels limit the site's growth to around 10 per cent.

"There simply aren't enough new users to make up for defecting ones after a certain point," said Martin.

"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."

Martin compared the site to Facebook and MySpace, which enjoyed similar levels of growth but retained many more users. As the sites became more mainstream, those retention rates grew to nearly 70 per cent.