Slyck reports torrent tracker The Pirate Bay achieved a peak of over 10 million peers on the more than 1 million torrents it tracks.
Let's consider these staggering numbers. 10 million simultaneous users represents a number never duplicated by any file-sharing entity. The largest P2P networks, such as FastTrack and eDonkey2000, both topped out with approximately 5 million users. Gnutella, fronted by LimeWire, is more difficult to calculate, however reasonable estimates place Gnutella's population among the P2P heavyweights. Despite these impressive stats, The Pirate Bay has managed to exceed all previous file-sharing populations.
10 million users is more than the population of New York City (8.1 million) and The Pirate Bay's home country, Sweden (9 million). To imagine the scale of The Pirate Bay's ubiquity, it would be similar to every person in New York City running a BitTorrent application and using The Pirate Bay as their tracker - and even then, there would still be 2 million individuals to spare. Skype manages to have similar ambitions to The Pirate Bay; as of the time of this writing, the VoIP P2P network has approximately 9.1 million individuals connected - but still short by 1 million users.
The Pirate Bay has also managed to break another achievement, as the BitTorrent tracker is now managing over 1 million torrents. Specifically, The Pirate Bay's statistics show that "10.044.335 peers" are currently connected, with "1.015.489 torrents" managed by the tracker.