A recent post from the Sysomos blog discusses the findings of a study they conducted focusing on the growth of Twitter and its stickiness as a service. This study was based on 1 billion tweets.
Summary of the findings below:
Twitter growth rate outside US is 8% which is higher than the 5% growth rate within the country.
Rate of contribution by new users has remained steady over past 4 months, suggesting continued user retention.
Users who have been using Twitter for over 9 months are becoming more and more active, reflecting strong stickiness to the service.