NPD tracks consumer file-sharing activity. It calculated that 14.5 million US households downloaded music files in April. In May the figure fell to 12.7 million, and dropped to 10.4 million in June, the company said today.
First idea that comes to mind is that RIAA campaign targeting swappers is working, although some say it could be paid services such as iTunes and BuyMusic grabbing a percentage of the music being downloaded, I see that possibility as unlikely.
On a related note, an anonymous California computer user went to court Thursday to challenge the recording industry's file-trading subpoenas, charging that they are unconstitutional and violate her right to privacy.