Let's talk about user numbers, shall we? I've got some interesting statistics to share, but before I talk about that we need to put things in perspective. Let's start with the Big Three: Nintendo, Sony, and Microsoft.
Assuming we're going to trust Wikipedia, then as of this writing the Xbox One has sold about 4 million units to the Playstation 4's 6 million. Those are pretty small user bases right now. I certainly wouldn't want to aim a big-budget AAA game at one of those two platforms as an exclusive. I mean, even if 50% of all Xbox One users buy your game, that's only two million units. Which is apparently not nearly good enough these days.
But it's early in the console generation, so let's look at the previous one. The Xbox 360 sold 80 million units. However, its stunning 54% failure rate (!) means that there's less than 37 million of them still in service. Sony might have bungled The Playstation 3 design, but it also sold about 80 million units. The PS3 failure rate was better than the Xbox 360, but still pretty embarrassing at 10%. Which means there are about 72 million still in operation.
I don't know if I feel comfortable declaring a winner for the last generation. Who do we give credit to? The platform that sold the best but had the fewest titles and gathered the most dust? (Wii.) The platform that sold well because units kept failing? (Xbox 360.) Or the platform that sold terribly at first but eventually caught up when the other two turned out to be poor long-term investments? (PS3.) I don't know. It's like a marathon where the first guy got lost and left the race, the second had a heart attack, and the third eventually crossed the finish line after two days. The last guy might have "won", but I wouldn't call him a winner.
Keeping all this in mind, I ran into an interesting figure cited at the Steam Dev days, showing that there were 75 million active Steam users as of the end of 2013. That number includes everyone who has bought at least one game and is currently active in the community. (So abandoned accounts and accounts with no games don't count.) That means there are more people actively using Steam than all Xbox, Xbox 360, and Xbox One users combined. Since a lot of consoles end up gathering dust or being used as video players, the number of Steam users is possibly greater than the number of PS3 and PS4 users combined.
This is huge. For the last decade the conventional wisdom has been that the PC doesn't matter. Publishers regarded the PC as a small market made up mostly of non-gaming machines, gutted by used game sales and plagued by pirates. Keep in mind that Steam games are, by virtue of being on Steam, not pirated. They also can't be loaned or re-sold. The hardware specs of these users is publicly known, and basically all of them are more powerful than an Xbox 360, and a lot of them meet or exceed the Xbox One specs. No matter how you look at it, Steam is everything publishers have claimed they wanted in a market. It has all the features (no used games, built-in DRM) that Microsoft was trying to put in the Xbox One, only done in a way that's less confusing for the end user. And most importantly, Steam is at least as big as any of the console platforms.
Whenever I talk about the dominance of Steam, I always get a couple of people accusing me of being a "fanboy". So before I go any further I suppose I need to make this clear again: I actually think it's terrible that one platform has such massive control of the PC market. I would have loved nothing more than to see Origin, Gamers Gate, or GameStop (née Impulse) become large enough to act as a counterweight to this growing Steam hegemony. You're free to love or hate Steam as much as you please, but if you want to be a grownup about this you have to be able to have a conversation about why company A is doing better than company B without reflexively shouting "hater" or "fanboy". This isn't about pride or fandom. This is business. Regardless of whether you love, hate, or are apathetic about Steam, its size and influence has it standing with the Big Three, which means that it is an important player in the industry.