well, prior to social media and streaming media, network and cable shows were in danger of being cancelled when their viewership dropped below 1 million viewers. Since the advent of streaming media, those standards have been lowered to now shows with 50K - 350K in the key demographic (24-50) are typically cancelled.
In CNN's case, those numbers are their primetime numbers, which is a big reason CNN is undergoing major org changes and content changes, because they've lost millions in add revenue per show.