NEW YORK – Neilsen Online has found that 60 percent of Twitter users quit the social networking application within a month of joining, a downward trend that experts say cannot help but limit the site’s growth over time.

“Let there be no doubt,” David Martin, vice president of Primary Research at Nielsen Online, wrote in a blog post, “Twitter has grown exponentially in the past few months with no small thanks to celebrity exposure. People are signing up in droves, and Twitter’s unique audience is up over 100% in March. But despite the hockey-stick growth chart, Twitter faces an uphill battle in making sure these flocks of new users are enticed to return to the nest.”

According to Nielsen, Twitter’s website had more than 7 million unique visitors in February this year compared to 475,000 during the same month last year.

The first chart below illustrates the problem, says Martin. “By plotting the minimum retention rates for different Internet audience sizes, it is clear that a retention rate of 40 percent will limit a site’s growth to about a 10 percent reach figure.”

Martin does leave room for the possibility that Twitter is still very new and could become stickier over time, but when he compares early retention rates with Facebook and MySpace when they were emerging phenomena (second chart), he found that they were twice as high as Twitter’s is now, and their retention only went up over time. Today, they both sit at nearly 70 percent.

“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. Frankly, if Oprah can’t accomplish that, I’m not sure who can,” wrote Martin.