And the pace of the fall off may be quickening. Last week’s performance show, featuring the songs of Mariah Carey, one of the most successful pop singers in history, was the lowest-rated Tuesday “Idol” in five years among TV’s most important demographic, adults ages 18 to 49. The subsequent results show, in which country warbler Kristy Lee Cook got the hook, delivered “Idol’s” worst Wednesday numbers among adults ages 18 to 34 since its first season back in summer 2002. Producers also saw depressed ratings for their “Idol Gives Back” charity extravaganza, which this year aired as a stand-alone show with no competition-related material.