WoWJutsu is a third party website that uses World of Warcraft Armory data to rank guilds. On it's front page it proudly proclaims 2.4 million total players ranked, of which 98% are said to have visited Karazhan. This has been widely interpreted as 2.4 million players having seen Karazhan, which is about half of the total population of 4.5 million US and Euro players. Other people said that this number seems to high, and various arguments went back and forward on who to interpret the numbers. The Wowjutsu FAQ isn't all that precise on who exactly is counted. I had started using guarded phrases like "according to the most optimistic interpretation of Wowjutsu data", because I wasn't sure which interpretation was correct, and constantly got attacked for my numbers being wrong. Enough of that! Lets find out how Wowjutsu REALLY counts raiders.
The trick to find out the truth is to drill down to a level of which you are absolutely certain. On the Wowjutsu site you can click on "Realms", choose US or Euro, choose your realm, and finally choose your own guild to see detailed stats about your guild. And then you can go to WoW and compare the Wowjutsu numbers with the WoW numbers. To be very exact in my case I even submitted a "manual update request" to Wowjutsu, so I have scanned Wowjutsu data of today from my guild.
The result: In WoW my guild has 394 members (characters, not players), due to most of us suffering from altitis. Of these 394 characters, 164 are level 70. On the Wowjutsu site my guild is considered to have 164 raiders!
Wowjutsu considers every level 70 character in a ranked guild as being a raider!
A ranked guild is a guild in which at least one person has Karazhan loot. But that doesn't make every level 70 character in that guild a raider! In the case of my guild I went through the rooster on Wowjutsu, which has detailed information on who has loot from what raid dungeon. I counted 73 players with at least one piece of raid loot. Due to my guild having Karazhan "on farm" and being generous with raid epics towards newbies, I do not think we have many raiders with no epic loot. Thus in the specific case of my guild, Wowjutsu overestimates our number of raiders by a factor of over 2. Wowjutsu counts us as 164 raiders, when in reality we only have 73 people who ever visited Karazhan. It even counts my warrior as a raider, of whom I am 100% certain that he never set a foot into Karazhan.
There are far less than 2.4 million players who visited Karazhan. Far less than 50% of the player base ever got the chance to raid.
By counting every level 70 character in a guild with raiders as a raider, Wowjutsu seriously overestimates the number of raiders in WoW. There might be tightly organized guilds in which every character is a raider. But in general more guilds are like mine, having a mix of raiders and non-raiders, and Wowjutsu counts them all as raiders. By extension that means that not only the assumption that 50% of players already visited Karazhan is too high, but also there are less people who have visited the Black Temple. Wowjutsu counts every level 70 character in a guild in which at least one person has Black Temple loot as having visited the Black Temple, and thus lists 7% of their 2.4 million "raiders" as having gone there. But there are certainly guilds in which people are at different stages of the raid circuit, and only the top guys have visited the Black Temple. Raiding remains far too inaccessible to WoW players, and the top level raid dungeons have only been visited by a far too tiny number of players.