Torrenting (using the BitTorrent protocol) is perhaps one of the favourite ways for internet users to download copyrighted material for free. It involves downloading a .torrent file and loading it into a torrent program, which then finds users around the internet (thanks to “trackers”) who have the necessary files stored on their computers . These “peers” then serve the files to the downloaders without a middle-man server, hence the peer-to-peer networking.
While they can be used legitimately, they are largely used for piracy. As of the time of writing, popular torrent search engine IsoHunt has indexed 8.25 million torrents with a combined size of 13,635 TB (or 13.6 petabytes). With 33.58 million connected peers, this gives a rough average of 4 peers per torrent. This also means the average size of a torrent indexed on IsoHunt is 1.64 GB.
Note: When talking about torrent file sizes, I’m not referring to the actual .torrent file size (which is usually a few kilobytes); I’m actually referring to the size of the files the torrent enables you do download from peers.
Here’s where it starts to get interesting. Public torrent aggregator Torrentz is currently indexing a larger 11.70 million torrents (without duplicates) across 33 domains, which provides for a good statistical base. Assuming the average torrent size is 1.64 GB, this means 19,188 TB of content is available through public torrents. If 99.86% of that is pirated, you have 19,161 TB of illegal content available compared to just 27 TB of legitimate material; this roughly means that for every 1 legit torrent, there are 700 containing pirated material.
Oh, and my assumption that 99.86% of torrents contain pirated material is based from the legit-only torrent search engine Mininova, which indexes a measly 16,686 torrents.
If we bring back the average of 4 connected peers per torrent and assume that 3 of these have full copies of the material tracked in the torrent, this means that stored across the globe is roughly 57,483,000 GB (57.5 petabytes) of pirated material that is accessible to anyone.
Of course this is not a complete representation of what pirated material is freely accessible to people: I haven’t counted private torrent trackers, newsgroups, file sharing sites and more, and the statistics I have are only rough estimates. It still shows though just how much content can be accessed through torrents.
On the more legitimate side of torrent usage, apparently both Facebook and Twitter use BitTorrent to transfer updates to their servers, and many games including World of Warcraft deliver patches and updates using the protocol.
Also, BitTorrent is used by some legal content distributors such as BitTorrent Inc. (for movies/music/TV) and Linux operating system developers (such as for Ubuntu and Fedora). Despite this, it's still highly dominated by illegitimate usage unfortunately, and many torrent indexers have had legal action taken against them for hosting torrents that could be used for piracy.