I found an interesting result about the usage of Twitter.
The table chart shown this page gives the result of a study by Net Ratings, originally released at 27, May, 2009. The given chart shows the usage and the character of the users of Twitter in the US, UK and Japan.
Overall, the users increased dramatically during last few months in every 3 countries. The country that had the largest users was the US, and the least was Japan.
In the US, Twitter had been becoming a common web service. According to the table, more than 1 of the 10th net users in the US are using Twitter. Compared with it, I should say that it is not so common yet in Japan: the Reach ratio is under 1%. Furthermore, the significant factors about Japanese Twitter users are their average minutes spent and the gender ratio. Usually Japanese users use Twitter less than 5 minutes, whereas that is more than 17 minutes in the US, and 27 minutes in UK. Also, 3 of 4th Japanese Twitter users are male, whereas in the US or UK the gender ratio is almost the half.
I heard recently Twitter is becoming more popular in Japan. I guess the gender ratio in Japan may become close to it in the US or UK in few months.