As part of Activision's earnings report that was made public today, the company states that Call of Duty: Black Ops 2 still has over 12 million monthly active users (MAU).
Momentum ahead of launch has been strong and engagement for Call of Duty: Black Ops II grew quarter-over-quarter again to nearly 12 million MAUs, an unprecedented level for a game that is three years old and only available on old-generation consoles.

This figure is actually about one million more than Activision's previous earnings report in August. During that period they stated that Black Ops 2 had 11 million MAUs.

Black Ops 2 came out in 2012. Since then we've had Call of Duty: Ghosts and Call of Duty: Advanced Warfare. It's also interesting to note that Black Ops 2 is only out on the previous generation of consoles (plus PC and Wii U), while both Ghosts and Advanced Warfare had the added benefit of spanning multiple hardware generations. That should put it into perspective just how much staying power Black Ops 2 has had over the past three years.