Twitter may be bringing in millions of new users every month—but it is struggling to keep them. Nielsen Online said Tuesday that Twitter’s audience retention rate, or the percentage of a given month’s users who come back the following month, is now a rather measly 40 percent. (It’s been at 30 percent for most of the last year.)

That’s about half the retention rate that Facebook and MySpace claimed at the same point in their lifecycles. Nielsen does not say why new Twitter users are dropping off at such a fast clip (We asked and will update if we hear back). Maybe it’s part of a broader fatigue with social networking sites (Who needs yet another site to check?) or frustration with Twitter’s simplicity.

But David Martin, vice president of primary research at Nielsen Online, says in a blog post that unless Twitter can boost the number of its users who stay around, the site’s growth will be stifled. “To be clear, a high retention rate doesn’t guarantee a massive audience, but it is a prerequisite,” he says. “There simply aren’t enough new users to make up for defecting ones after a certain point.”

Of course, at the moment, that’s not a big problem for Twitter, which doubled its size last month, more than offsetting the loss of any folks who abandon the service. But it’s an indicator to watch as some of the early-stage buzz around Twitter naturally wears off.

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