A controversial US study of 43 major file-sharing sites found they fielded more than 53 billion visits a year.
MarkMonitor, author of the study (pdf), drew a connection between file-sharing and piracy that raised the hackles of those named, one of which threatened retaliation through defamation action.
North America and Western Europe were fingered as the host locations for 67 percent of the largest digital piracy sites, based on traffic data from Alexa.com.
The largest three file-sharing sites - RapidShare, Megavideo and megaupload.com - each generated more than 13 million daily visits on average, collectively generating more than 21 billion visits a year.
MarkMonitor acknowledged sites had takedown processes for disputed content but said they didn't go far enough.
The Swiss site last week won its action against a German court ruling that it took insufficient measures to stop the distribution of Atari's Alone in the Dark PC game.
And RapidShare disputed MarkMonitor's classifying it as a digital piracy site: "This defamation of RapidShare as a digital piracy site is absurd and we reserve the right to take legal action against MarkMonitor".
"RapidShare is a legitimate company that offers its customers fast, simple and secure storage and management of large amounts of data via our servers," it said.
Video community Megavideo's terms of service prohibited users from submitting material that is "subject to third party proprietary rights ... unless you are the owner of such rights or have permission from their rightful owner".
Megavideo was owned by Hong Kong company Megaupload, a file-sharing and hosting service that is the 68th most visited site in the world, according to Alexa.
Alexa ranked file-sharing site Pirate Bay 89th most trafficked in the world. Last year, three Pirate Bay founders were sentenced to four to 10 months in prison for copyright infringement.
MarkMonitor said it used patented technology to identify suspected counterfeit or piracy sites, before filtering those results and examining by hand more than 600 results to determine classification.