Among shows in 2015, Season 6 of AMC's "The Walking Dead" led ratings in key demographics and spurred major viewer engagement on social media. 

It might come as no surprise that the show led the ratings in all key demographics, translating into massive advertisement revenue for the network and a guarantee for multi-season renewals for the show. According to Variety, "The Walking Dead" led the pack in the key demographics of adults 18 to 49 and adults 18 to 34.

But it did not win in total viewers, falling behind "Sunday Night Football" on NBC and CBS's two powerhouse shows "The Big Bang Theory" and "NCIS."

These numbers were provided by Nielsen, who also measured social media engagements for various shows on TV and found "TWD" to be No. 1 on Twitter.

Despite ranking No. 4 in total viewers, the show's No. 1 ranking in key demos gives it an edge with networks, who look closely at those numbers in order to measure success. Those key demographics are the ones that most advertisers want to reach for their products, and the numbers show that "TWD" is a powerhouse that never fails to deliver important viewers.

Fans of the series should also know that a show doing this well on a cable network is rare, when compared to the big four broadcast networks, which have historically dominated the ratings due to their reach.

This is also a key consideration moving forward, given that so many U.S. cable subscribers have recently been "cutting the cord" and opting for streaming and broadcast antennas, due to the consistent rise of cable package prices.

There is reason to believe that the market is about to experience a major shakeup in the near future, and ratings like these will have to be alternatively measured for audiences who opt to stream movies and TV shows instead of watching them live.

There are no measurements provided by streaming companies like Netflix and Amazon. "TWD" currently streams its seasons on Netflix under a contractual deal with the company, but the episodes appear months after the show airs on AMC.