The RIAA warns over 1 million copyright infringers each year

"In less than two years the RIAA [Recording Industry Association of America] has sent copyright infringement notices to 1.8 million Internet subscribers and 269,609 to colleges and universities. Despite this staggering average of more than a million infringement notices every year from the recording industry alone, the effect on file-sharing levels seems unnoticeable.
For years, content owners such as record labels and movie studios have been sending copyright infringement notices to Internet users. They hire companies such as DtecNet and BayTSP, who monitor file-sharing networks and automatically send infringement notices to Internet providers. The Internet providers on their turn are legally obliged to forward these to their customers."
Source: TorrentFreak, 4th July 2010