Activision has stated that Ghosts is the #1 selling game on the Xbox One and I don't doubt it. It's familiar. I have it as well as Forza, LEGO Marvel Super Heroes and Madden 25. Spend most of my time on Ghosts and Forza, though.
And while there have been well over a million One's sold you have to realize that, even if half of the people purchased Ghosts you're still only talking about maybe 600,000-800,000 people owning it. And of course they all aren't going to be online at once.
Think about it. BO2 sold over 24 million copies. You figure around 10 million or so for the 360. So you have roughly 10 million 360 users with the game and yet regularly saw around 300,000-400,000 users online at once. Divide the number of users by ten for the One and then you have around 30,000-40,000 people online at any given time. What I'm saying is the averages are pretty close to the same.
Anyway, as others have stated I'm sure that just after Christmas we will see an influx of new One users and hopefully the numbers grow to the 50,000ish range. Probably won't approach 360 type numbers (300,000-400,000) for another couple of years, though.