I found two studies from 2010: one by a Princeton student[1] and one by the University of Ballarat (Australia) [2].
Both of then found that the vast majority of bittorrent traffic, not surprisingly, is illegal and both were heavily critiziced.
I quote:
"...89.9 per cent of them were illegal and, of the remainder, 8.2 per cent was porn."[2]
"Overall, about one percent of the total files were categorized as "likely noninfringing."[1]