Micro-blogging service Twitter has seen phenomenal growth over the past year. Traffic to the site has skyrocketed. Major news organizations including CNN and NPR have started to use Twitter to offer news and information. And a few high profile people including Shaquille O'Neil and Oprah have signed up for the service.

But while Shaq has become an active member of the Twitter community, Oprah used it for a few days after talking about Twitter on her show. Her last update was posted nearly a week ago and it looks like she may have abandoned Twitter. And recent research shows that Oprah's experience may actually be far more common than Shaq's.

According to Nielsen Online, more than 60 percent of the people who sign up for Twitter fail to return to the site a month later. So while the number of registered Twitter users is on the rise, the number of active users is climbing at a much slower rate. As Nielsen puts it, "there simply aren't enough new users to make up for defecting ones after a certain point."

Of course, Twitter is still new and it's just starting to break into the public (non-geek) consciousness. So there's still plenty of room for the service to grow. But new users will have to find a reason to stick around.