Oh well...itunes is not so used by people in Japan. But Recochoku is more impressive.
Recochoku is a similar thing to itunes..but it's more like for mobile phones. Downloading music for mobile phones etc. Al lthat stuff, like ringtones etc.
I know that iTunes isn't the biggest music retailer in Japan, but the #1 song is selling 50% of what the UK #1 (on iTunes) is selling.
iTunes is not THE best indicator but it's surely not irrelevant.