Bittorrent is a market where the 'bit' is the medium of exchange. BitTorrent is a market in which barter has proven more efficient than money-mediated transactions.
Users return contribution, i.e. leaving their connection open for others to use, is calculated through a 'share ratio'.
"Because BitTorrent relies on the upstream bandwidth of its users—and the more users, the more aggregate bandwidth is available for sharing the files—it is considered good etiquette to leave one’s BitTorrent client open after downloading has completed so that others may continue to gain from the file that has been distributed…
Some clients also report the “share ratio”, a number relating the amount of data uploaded to the amount downloaded. A share ratio of 1.0 means that a user has uploaded as much data as they have downloaded. A share ratio greater than 1 means that a user has uploaded more than they have downloaded. It is generally considered good form to at least share back the equivalent amount of traffic as the original file size. Share ratios are more important on BitTorrent than they are on other peer-to-peer file sharing networks, because many BitTorrent trackers require users to maintain a minimum global share ratio. On some trackers that require users to register, the minimum global share ratio may start at around 0.5 and increase over time, so that the user has adequate time to upload and share their files. Users with a share ratio below the minimum may be put into a restricted “upload-only” mode, where they may not download until their share ratio reaches the minimum." (http://en.wikipedia.org/wiki/BitTorrent)