Right, but I was speculating on the continued growth of Steam in terms of gaining new customers. "Active users," as Valve defines them, don't necessarily spend a significant amount of time playing games, nor are they customers, or even actual individuals - that chart's title is misleading because the numbers are still a measure of active accounts, not users. I'll add that if that chart were to address the equally important question of "Why" in addition to the "How," I would be surprised if the single largest driving force behind the significant increases of the past few years wasn't DOTA, which is, as we all know, a free-to-play. There are tens of millions of MOBA gamers ripe for Valve's taking, but just as we can't assume an equal ratio of Steam accounts to Steam users, neither can we reasonably expect all of those who play free-to-play games (who are, again, almost certainly the single greatest force acting upon recent growth) to be customers.
Make no mistake though, the number of active Steam accounts will increase every year for the foreseeable future. This number is prone to misinterpretation, however. For example, those who have used the number to draw hasty conclusions about the comparative success/popularity of Steam relative to Microsoft and Sony in the context of the next-generation console launch last year. To be clear - those individuals are comparing the rate of creation of free Steam accounts to the rate of purchase of $400 devices, an apples-to-oranges comparison, and drawing conclusions from it. Yikes.