Well, this article, dated April 24, 2008 says that the PirateBay tracker had 12,000,000 peers. This is over a year ago - you can imagine what that number is at now!
http://torrentfreak.com/the-pirate-bay-smashes-12000000-bittorrent-users-080424/
In 2008, uTorrent (a bittorrent client) reported 28 million unique monthly users.
http://forums.techarena.in/web-news-trends/1093373.htm
So, the number of users is the the millions, which isn't surprising at all.