Let's take a look at the 2011-12 season (the seventh overall) of Supernatural on the CW.
Rating the Ratings: The second season of Supernatural on Friday did not go nearly as well as the first. It's kinda crazy that the show dropped more year-to-year this season (when it stayed put but lost the Smallville lead-in) than last season (when it moved to considerably lower-viewed Friday). But a pairing with Nikita just wasn't quite the same, probably in large part because Nikita only had about half the audience of Smallville. So of course it was the show's worst season yet that led to the CW finally upgrading Supernatural's timeslot. It heads to Wednesday to air after high-profile newbie Arrow next fall. Grade: D+.
Here's the now updated War of 18-49 post for Supernatural.