I'm not the most plugged in person to the Japanese Internet, but over the last 6 months or so I've started to see Twitter rise pretty fast here. First it was on the sidebars of the portals, then the Japanese word for "tweet" started to show up in the headlines for the scandal rags that I scan on the train ride to the office, then folks at my office started asking me whether I had a Twitter account yet. So you might be seeing some growth in addition to possible sampling issues.