From a recent Tweet we now know that Modern Warfare 2 has had approximately 25 million unique players online. Now tell me if my math is wrong, but: 
 
25 million unique players ~ 25 million copies of the game 
25 million copies - (number of copies sold on 360 + PS3 from VGchartz info) = PC copies sold
25 million - (9.8+7.05 million) = 8.15 million copies 
PC copies sold = 8.15 million  
8.15/25x100% = 32.6% of all sales/players (assuming that buyers on separate platforms have approximately the same attachment)
 
That's all well and good, but seems to lie in direct contradiction to the report that only 3% of sales were on PC: http://news.softpedia.com/news/Only-3-Percent-of-Modern-Warfare-2-s-Buyers-Were-PC-Gamers-127675.shtml       

What's the explanation for this? Are Vgchartz's numbers really old? Are the number of PC gamers being significantly underestimated?

A:  No it didn't.
 
Second hand sales = 1 sold copy but multiple unique players.

I own the game and have played it = 1 unique user for 1 copy.
I lent the game to my friend for a week to play it = 2 unique users for 1 copy.
I have now traded the game in and presumeably someone has bought it and played it online = 3 or more users have played my single copy of MW2.
 
Unique online players doesn't really mean anything, it's just a big number to impress people.

" A:  No it didn't. Second hand sales = 1 sold copy but multiple unique players. "

25 million unique players does not come close to 25 million copies of the game  because it doesn't include the abundance of players using used copies or the number of households which contain multiple unique players sharing a disc.  Siblings, father/sons, dorm roommates, etc.      

interesting

The number of pirated copys is also something to consider.
 
Plus, one copy in a household with more than 1 gamertag (siblings, roommates, etc).
 
25 million unique users means only that. Don't read too much into it.

8 million? when i look at the online player count i only see 50-100k online. So i doubt that

its the PC version, so i guess around 50% are pirated

" its the PC version, so i guess around 50% are pirated "

I'm guessing the PC version sold around 1 - 2 million copies.

Considering that it was the on-off  top seller on Steam for 2-3 months, and its still in the top 10.

I have a hard time believing that.

Its not like we'll ever truly know since Steam does not release their sales data...