The popularity of Call of Duty probably comes as no surprise, but it seems to have been a smart move to relaunch Call of Duty on Steam. Recently, the latest iteration, Call of Duty: Modern Warfare II, broke a record for the series, with 263,174 players at one time (thanks, SteamDB). As of writing, over 112,319 unique users are playing against each other at any one time. While not the biggest on Steam, it's big for Call of Duty, as Black Ops III held the previous record of 63,681.
Are you playing Call of Duty Modern Warfare II?