They've monitored online activity and numbers have been "peaking consistently" at around "20 players or less." All Halo 2 network services for PC will terminate as of February 15th.
Halo 2 was the last in the series to be ported to PC. It famously demanded users have Windows Vista because of DirectX, but a third-party patch let it play on XP.
"...we’re sad to report that the service end-date for Halo 2 PC Multiplayer will be February 15, 2013. We’ve been monitoring the population for months, and it’s been peaking consistently at approximately 20 players or less," 343 via Halo Waypoint. "Those that own this game can continue to enjoy Multiplayer over LAN after that date; however our network services will be turned off at that time. We love you, Halo 2 PC, and you will live forever in our hearts."
The Halo franchise has generated in excess of $3 billion in sales for Microsoft, selling over 46 million units.