Considering the rate at which the mod is growing in popularity, it seems inevitable that the number of unique users will reach the 1,000,000 mark soon enough.
In the last month the number of unique users have risen by 500,000 which is a pretty big number. This is probably due to the recent Steam Summer Sale, which ArmA II: Combined Operations was a part of.
DayZ is an online open world survival horror mode for ARMA II which was released back in 2009. The mod features some interesting game elements. To be short you’ll need to think more of scavenging for food and supplies than shooting hordes of zombies with state of the art fire-arms.