SYDNEY (Reuters) - Today's Twitters are often tomorrow's quitters, according to data that questions the long-term success of the latest social networking sensation used by celebrities from Oprah Winfrey to Britney Spears.

Data from Nielsen Online, which measures Internet traffic, found that more than 60 percent of Twitter users stopped using the free social networking site a month after joining.

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

"For most of the past 12 months, pre-Oprah, Twitter has languished below 30 percent retention."

San Francisco-based Twitter was created three years ago as an Internet-based service that could allow people to follow the 140-character messages or "tweets" of friends and celebrities which could be sent to computer screens or mobile devices.

But it has enjoyed a recent explosion in popularity on the back of celebrities such as actor Ashton Kutcher and U.S. talk show host Oprah Winfrey singing its praises and sending out "tweets" which can alert readers to breaking news or the sender's sometimes mundane activities.

President Barack Obama used Twitter during last year's campaign and other prominent celebrities on Twitter include basketballer Shaquille O'Neal and singers Britney Spears and Miley Cyrus.

Twitter, as a private company, does not disclose the number of its users but according to Nielsen Online, Twitter's website had more than 7 million unique visitors in February this year compared to 475,000 in February a year ago.

But Martin said a retention rate of 40 percent will limit a site's growth to a 10 percent reach figure over the longer term. "There simply aren't enough new users to make up for defecting ones after a certain point," he said in a statement.

Martin said Facebook and MySpace, the more established social network sites, enjoyed retention rates that were twice as high and those rates only rose when they went through their explosive growth phases.

Both currently have retention rates of about 70 percent with Facebook having about 200 million users.

"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," said Martin.

(Writing by Belinda Goldsmith, Editing by Miral Fahmy)