It wouldn't surprise me if there were amillion relatively active players, but think about this:
200 million rounds / 5 million players = 40 rounds per player
I've been here less than a year and I have played 550 rounds. Many, many players have many, many more rounds than I. So, I'd guess a great many of those 5+ million have only played once or never even completed a round. Or only play alt, MP, etc.