Steam is without a doubt THE biggest gaming platform around. How big?
14 million users big.
The distribution portal hit it big on the 7th of January, with a peak milestone of 14,207,039 active accounts. Of course, this isn't necessarily active players - the number includes anyone with an account open in the background. But it's not a number to sneeze at.
Other cool stats include the not all that surprising fact that DotA 2 is the top game, with 950,000 concurrent users, followed by CS:GO and GTA V at 675,195 and 116,230 users respectively.