Have you seen the Nielsen report entitled "Twitter Quitters Post Roadblock to Long-Term Growth?" It's definitely made the rounds. And if your inbox is anything like mine, you have a lot of people asking questions and raising concerns over what this means.
It means nothing, the numbers are misleading.
Here is why. Twitter is not a social networking site like Facebook or MySpace (comparison from the article). The question is really how does someone use Twitter? Since Twitter is used differently than those sites, it may be better to compare it's use to Google reader or NetVibes. For all of the hype around Twitter, it is a broadcast system. The trick, then, is attracting and retaining followers. And that is depend on your content (and the content of others)--not the platform. In counseling clients, I would say that the stat to watch is how many people link through your tweets and how does that compare to PPC click-through, email read rates or display ad performance.
And then there is the mobile component of Twitter. Even though Facebook has an iPhone app, no other platform of this size is so portable. With the 140 character limit and open API, Twitter makes a lot of use of 3rd party clients, text messaging, and a myriad of mobile applications. In looking at Twitter usage patterns, many Twitter users start off using the web site interface, then quickly migrate to a third-party applications on the web and through mobile clients. Take a look at this graphic: (full list here)
Perhaps Nielsen did not take this into account for the full story. But either way, this is not the end of Twitter. If you promote your Twitter feed properly, tweet good content, and find value in its use-- you get a lot of conversation started, linking to relevant content, and gather insight that helps you or your company--then retention rate of the web site alone is not a measure of whether you should use Twitter in your mix of communications tools or not.
After all, this is social media. It wouldn't be any fun if this stuff was 100% predictable, consistent, and stagnant. (Though the ad guys would love that, wouldn't they?!)