Many torrent tracker sites compete. Thus, they publish data such as number of available torrents, of registered users, of seeders and leeches. One of the most important sites, mininova publishes the number of downloaded torrents: 4.918.964.636. At their current pace, mininova will reach the threshold of 5.000.000.000 downloaded torrents in a few days.
I find this figure more interesting than the other ones. For instance, the number of available torrents is not really meaningful. Many torrents are not active (thus the health bar on any site). Mininova publishes other data. The distribution of the type of downloaded contents is interesting. 39% are on TV series, 22% on movies, and 20% for music. The most downloaded torrent is episode 17 of Heroes’s first season. This craze torrents of TV series is extremely interesting and should be carefully analyzed by broadcasters.
In any case, BitTorrent is really the protocol of choice. Many progresses have been done both by the software themselves and by tools allowing search (BitCHe, TorrentFinder toolbar, …), making them easier to use.