Unlike most other websites, the success of Twitter cannot be measured by the number of uniques. Thanks to the third party Twitter clients. Then, the only way to measure its success is via the number of tweets.
In a recent study, Sysomos looked into the growth of Twitter month-over-month since the beginning of 2010. According to this study, users contribute close to 53 million tweets everyday. This is up from the 50 million mark observed a month ago.
Here are the monthly growth in Tweets for US and non-US demographics
Jan ’10 : 10%
Feb ’10 : 4%
March ’10 : 5%
Jan ’10 : 13%
Feb ’10 : 9%
Mar ’10 : 8%