Most people who use Twitter will be gone within a month according to newly published figures. Number-crunchers say the trend means Twitter’s past growth rate will be unsustainable.

The figures from web research firm Nielsen Media say that only 40% of people using the site during a month will still be using Twitter the following month. The firm’s David Martin says this retention rate is too low to allow Twitter to continue to grow significantly: “There simply aren’t enough new users to make up for defecting ones after a certain point.” He says that, based on the experience of other membership websites, Twitter will struggle to pass the point where 10% of internet users are members of the site.

Nielsen also noted that the retention rates for rival social networking sites Facebook and MySpace were constantly higher as they developed; when each had an audience the same size Twitter has now, 70% of users were sticking around from month to month (see Nielsen Media image below).

Perhaps the most surprising note from the figures is that for the 12 months leading up to Oprah Winfrey’s publicizing of the site, the retention rate for Twitter was just 30%. Logic would suggest an influx of users who have only just heard about the site through such a mainstream source would be less likely to stick around, not more likely. It could be that those who came in the Oprah bandwagon, reportedly in the hundreds of the thousands, haven’t had enough time to become bored with the site yet.

It’s worth noting that the retention rate covers all users, not just new members. So while the most likely explanation for Twitter’s low rate is new users joining up, taking a look around and getting bored, it’s also possible that established users may be quitting because they feel the site is losing its exclusivity.

There have been some questions about how the figures were put together. If, as some suspect, the figures simply come from visits to the Twitter website, they could be misleading: many users, particular established ones, use standalone applications to post and read Twitter messages.