An article I read a few months ago quoted real figures from the UK charts. It said that particular weeks number one single was the worst selling number one single of all time (which had happened a few times recently), selling approx 24,000 units (can't remember exact, but it was mid twenty thousands, around 23 to 24 I think). The article went on to say that up until the mid to late ninties, sales for number one charting singles were generally well over 200,000 units.
I am not saying this is completely attributable to downloads. I am saying it is obviously largely attributable to piracy, with downloading being a large part of piracy.