I'd refer to a game as "dead" if the community is small enough that ya've memorized every player within a week of playin' it regularly. ohai AC:III on Wii U
In other words, once a game completely losses public interest I'd consider it dyin'. Once the community comprises of nothin' but the occasional few newbies and a high percentage of dedicated veterans, it's dead. Said community will no longer grow and only get even smaller over time.