Earlier quoted context omitted.
http://torrentfreak.com/the-pirate-bay-smashes-12000000-bitt... That's from 2008 announcing Pirate Bay had hit 12 million concurrent peers on their tracker. That's not total users, that's total online. Take that number and multiply it by 5 to 10 to estimate total users, double it to account for the rest of the global trackers (low?) and increase it by some percentage to account for 3 years of growth and you're in the…
By having 12 million concurrent peers on their tracker, aren't they pretty much de facto centralized even if the protocol is theoretically decentralized? If The Pirate Bay disappears tomorrow do all of those users just switch over to another tracker? Very likely not. They'd just wait around to hear about the next Napster->Kazaa->LimeWire->Pirate Bay from their tech savvy friends, remaining mostly oblivious to the tec…
Even when they did use a tracker it's probably more reasonable to think of it as a directory server more like LDAP or such. At their base even classic decentralized protocols like SMTP rely on DNS as a directory, IP on routing tables etc.