I ran a file sharing site for a while (it started out as a way to get around email file attachment limits). Sure enough the child pornographers found out about it and started using it but close collaboration between the dutch child porn taskforce and myself meant that using the service for child pornography was a huge risk to the uploaders and downloaders alike.

The police never cared about any potential copyright infringement (which I'm fairly certain must have occurred as well simply because we could not possibly monitor all the uploads manually).

Eventually I clued in to a way to really close the door on the childporn if we could collaborate even more tightly but before this could get off the ground my main contact at the dutch police force died and his replacement did not show much interest (there was a huge flap with child porn made in daycare centers here at the time so I can see why he would have been too busy).

Absent the collaboration I shut the service down, but in principle I think file sharing services have many good aspects, they are just tools and can be used for good and bad alike.