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]

[1] http://arstechnica.com/business/2010/01/bittorrent-census-ab... [2] http://arstechnica.com/tech-policy/2010/07/only-03-of-files-...