I assume there are currently 7m - 10m subscribed accounts in wow, so lets assume the lower of the 2.
1% of 7,000,000 = 70,000 people
I haven't seen any statistics from MMO-Champion themselves, but I would find it hard to believe there's 70,000 regular posters here. I could believe there are at least 70,000 regular lurkers who read posts, but not take active part in discussions. Number of accounts probably isn't reflective either (for example I read the latest news most days but I only have a small number of posts so most of my opinions aren't shared)
(Also worth stating I filter MMO-Champion to only show WoW, Class Forums and D3 forums so i'm judging it on that)
Look at how many players there are compared to the relatively low traffic these sites get, and most of that by repeat posters/visitors....i'd not be surprised at all if it was around 5% or so, i played and raided for years without ever once knowing there were these sites online, and i was nowhere near alone then and was still plenty knowledgeable to raid end game. These sites are fun but by no means necessary.
I can't venture a guess on an exact number, but I imagine it's a very, VERY small percentage. I always find it pretty funny when people say stuff like "[insert class here] needs to be nerfed, 90% of WoW players agree with me!" and base that on maybe 10 or so "yeah, I agree you're right!" posts on the forums.
A large proportion of the players I've spoken to in game over the years don't visit forums. Of those that did, most of them told me they either only visited Blizzard official forums or that they only read MMO-C front page, and don't visit forums to post, although they may occasionally read.
Of course I had a small sample size, but it has definitely led me to believe that the people on this forum are in the minority as far as the playerbase is concerned.
If the sample was random enough, the amount of posters on here would be more than sufficient to draw accurate conclusions from.
The very act of actually going to a fan site and posting on a forum disqualifies it from being a random sample. I agree with Shamanic that the percentage is much, much lower than people imagine, probably less than 2%. And a certain percentage of those--especially here--aren't interested in posting anything which is really useful or have been out of the game for months or years. Which is not to say there isn't value to be had in discussing the game with people and getting a general sense of how people feel about something. But everyone plays differently and the only way to really know about something is to and experience it for yourself.
But I can verify myself that stuff that is common knowledge about upcoming things in the game is really hardly known by anyone. I mention upcoming class changes from the front page news in guild chat all the time and it's always new information for practically everyone who hears it.
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