People are joining Twitter in droves, but far less than half of them are sticking around more than a month on it. The blogging site that requires users to share thoughts in 140 characters or less has a retention rate of just 40 percent, said David Martin, vice president of primary research for Nielsen Online. That pales to other social networking sites like MySpace and Facebook, each of which have retention rates around 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," Martin said in a blog post. "A high retention rate doesn't guarantee a massive audience, but it is a prerequisite. There simply aren't enough new users to make up for defecting ones after a certain point."

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