PC gaming includes more then one platform though. Where as for consoles there is the Wii, PS3 and 360. The PC has desktops, laptops and tablets that games can be played on, so there is more then one platform (or device). PC gaming has grown over the last 3 years. A lot of it is due to cheaper digital DLC and social network applications. MMO's are still king on the PC. There are 100's of MMO's out there.

Actually when we look at some multiplat games from the past few years, the PC versions are usually on par or below the PS3 versions. (Borderlands comes to my mind, Dead Space) and pretty much always lower than the 360 versions. (Bad Company 2, Mass Effect 2...) Now after the slim, PS3 software sales have risen to 360 levels.
http://fadellc.com/press_17.html

While PC exclusives can also have very high sales in the past few years (Half-Life 2, WoW, Starcraft 2, Sims 2) they still can't compare with the heavy hitters of Wii and 360.
(EDIT: haha just noticed that I mentioned a lot of last gen games in this paragraph.)

So no I don't agree that PC is the biggest platform at least in terms of revenue. Probably not even the 2nd biggest and after the slim effect, it's likely to assume that it's in last place now.
Biggest platform by amount of active gamers ? I guess so, but how many of them are pirating the games ? A lot it seems or else software sales would be much better.

Yes, you could consider desktops, laptops and tablets different HW platforms, and different SW ones too, but while this could be a problem in the period between Win95 until Win2000 included, since XP, its user base become fastly incredibly huge granted that XP itself became a unifying architecture with HW and SW support and compatibility granted even when successive versions of Windows were released, and Vista's disappointing sales compared to it, made it even stronger. So since XP, a unified Windows PC platform actually exists (also against MS wishes that would like more people to upgrade to successive versions, while most users happy with XP don't want to drop it until it's supported). It can be argued that also Win 3.x was a unifying Windows PC architecture for many years, more than 5, but back then Windows had a small user base compared to what Windows achieved later and it was almost irrelevant as a gaming platform compared to DOS on PC and to the most successful consoles of its times. But I agree that for what regards genres and styles of gaming, PC always was quite defferent from consoles and it is even more now, more recently also regarding business model, up to a point that direct comparisons don't make much sense (except maybe with XB360 during the first half of its life, but more with XB1) if not for mere sake of comparing sales volumes and revenues of HW and SW.

In fact I meant mostly about HW and SW sales volumes. Piracy is higher and gross revenue is maybe lower for SW (but higher for HW), but dev costs are lower too, and there isn't the royalties standing charges. about the size of heavy hitters taken individually, you are totally right, no PC game ever reached the sales of XB, XB360, PS2, DS and Wii best sellers and very few PC games even reach PS3 best sellers, but it must be said that on those consoles too their few best sellers are out of reach for their other games. BTW I wrote in the same order of magnitude of the best as totala sales across a generation, because PC enjoys a more constant sales performances, while consoles start, reach a peak (or two like this gen with consoles getting rejuvenated at their mid-life) then they decline and are outsold by PC in their late years, and I also specified that particularly strong gen leaders like PS2 and DS, far exceeding 100M units sold lifetime, beat PC anyway from any point of view, not only SW gross revenue. But as I wrote answering XanderZane, PC market already was very different and it is even more now, so the comparison makes sense just for sake of comparing HW and SW unit sales of huge but different markets.

But that PC SW is hugely undertracked is totally, absolutely true!!!