SAN FRANCISCO — Twitter quitters outnumber the flock of habitual tweeters on the rapidly growing online communications service, a new study suggests.

Most people aren't joining the Web site's jumble of conversations for very long. More than 60 percent of Twitter's U.S. users don't return a month later, based on an analysis of traffic trends unveiled this week by the research firm Nielsen Online.

The lackluster retention rate of 40 percent suggests many people don't see the point in spending time on Twitter, which allows anyone to write about what they're doing or what's on their mind in messages, or "tweets," limited to 140 characters.

While some of the chirping is entertaining, thought-provoking or just downright helpful, much of the chatter can be quite banal as people update when they are eating, drinking, puking and even defecating.

It's possible Nielsen Online is exaggerating Twitter's retention problems because its tracking tools don't account for applications on mobile phones and other devices that make it possible to tweet and read messages without going through the Web site. That limitation means Nielsen Online could be wrongly characterizing some active users as Twitter quitters.

Twitter co-founder Biz Stone didn't immediately respond to a Wednesday request for comment about Nielsen Online's findings.

Despite the defectors, Twitter is amassing an impressive audience. In March, the San Francisco-based service attracted a U.S. audience of 13.9 million, an increase of more than 25-fold from roughly 500,000 users at the same 2008 juncture, Nielsen said.

Given that Twitter received an endorsement from Oprah Winfrey on her television show earlier this month, the April numbers will likely be substantially higher.

A high turnover rate isn't unusual when a young Web site starts to catch on, but three-year-old Twitter is losing users at a quicker pace than other popular online hangouts like Facebook and MySpace did at comparable stages of their growth, according to Nielsen.

Both Facebook and MySpace were retaining about 60 percent a few years ago and now are holding on to about 70 percent of their traffic.

"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," David Martin, Nielsen Online's vice president of primary research, wrote in a blog posting about his firm's findings.