Game Statistics

[TIGR]

What's the best way to compare the number of people playing different games?

In this case I'm particularly interested in comparing the number of people who play multiplayer CoD 4, MW2, Black Ops, and MW3.

From Steam we have the game stats page which shows only the current day's current and peak players:
http://store.steampowered.com/stats/

From XFire we have game rankings, and for each game, a graph of the hours played per day, each day of the past month along with the users per day:
http://www.xfire.com/games/

These two sets of stats aren't directly comparable and I expect each has bias (XFire toward CoD4 players, Steam toward the rest). There are other possible sources I'm not taking the time to list here, each with its problems. Is there some obvious was of getting directly comparable numbers of players for each game that I don't know about?