The always insightful Stuart Dredge over at MobileEntertainment has crunched some numbers on Game Center's actual popularity. Since Apple's own gaming social network on the iPhone launched, I've heard lots of different things from developers in terms of how they're implementing and using it. Some developers jumped in head-first, bringing all of their apps on board quickly, while others have only brought one or two apps on board at a time, opting instead to wait and see what their users are into. And some developers haven't even started on it yet, instead waiting to see how adoption works.
So, in order to get something resembling hard numbers, Dredge took the reported sales of a few popular App Store apps, and then he compared those to the number of players listed as playing them in Game Center. You can see the top few examples above -- keep in mind that this is a picture painted with a roller rather than a fine brush; it was just created to give an idea of what's happening rather than exact figures.
But there are some conclusions to draw -- a title like Angry Birds, with over 6 million players, is only seeing about a quarter of those users using the Game Center integration. At the same time, a newer title like Cut the Rope has a majority of its players using Game Center. In other words, Game Center is still in the early adopter stage -- the mass market of iPhone users hasn't yet jumped on board. But users who seek out and find the latest and greatest apps (and Cut the Rope is a hit that's only a few weeks old) are signed up and playing on Game Center already.