In a farewell for fans, Bungie has released a dump of Halo data since the game launched in 2004. Across the series, 20,880,250,123 games have been played online, logging 235,182 cumulative years of playtime, and 136 billion kills. Halo 3 (and the ODST expansion) accounted for almost half of all games played at 11.5B. See all the stats here.