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.

"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.