According to Steam Stats DotA 2 peaked at 147k players today, that's a game in Closed Beta. Granted it's the most popular game on Steam, but so is MW3 the most popular game on consoles. The difference as I see it is that games on Consoles are either packed, or empty. MW3 may have 175k people playing, but I guarantee most games other than FIFA or Madden are nearly empty. Companies shut down servers for Console games like a year after release because there are only a few hundred people playing. Compare that to Steams peak users for the day;
DotA 2: 147k
TF2: 102k
CS1.6: 57k
L4D2: 49k
Borderlands 2: 40k
That's the top 5 games for the day, and it amounts to 395k people. The peak amount of users online for Steam alone of the day was 5.4 million. Hell, the top 10 games only account for 566k users. MW3 on the PC had a peak of 35k users today, which isn't all that bad considering it was a pretty lame console port, and according to VGChartz only sold 1.52m copies (the PS3 version did just under 12.5m and the 360 version did about 14.6m).
Now, this is only Steam. This isn't factoring MMO's, Origin users, or games that do not apear on Steam (SC2 and Diablo 3 come to mind). So needless to say, I highly doubt Xbox Live or PSN have more people actively playing daily than the PC does as a platform, and there is a big reason for this. Platform Diversity. That is, with a PC, you can play any game that is made for PC's, with Consoles, you can only play games made for your console. Imagine if PSN and XBL were combined, that's basically what the PC is. If you're playing MW3 on the PC, you're playing against everyone who has a PC Version of the game, not just the people with a 360 version or PS3 version or Wii (LOL) version.