First and foremost, the survival of television series these days is heavily determined (as it has been for the past half century and more) by the monolithic Nielsen Company that reports the ratings which measure audience size. The primary number focused on these days is the rating in the 18-49 demographic because that is the group that most advertisers target and these sponsors rely on the measurements to determine which shows to buy advertising time on. You can read a bit more about how the ratings work at my FAQ and also over at TV by the Numbers. Read the full article at CancelledScifi.com.
Also see: Is There Still Hope for Syfy’s Haven? Can the Fans Keep the Show Alive?