According to TVByTheNumbers.com, the WrestleMania 28 special that aired on Saturday night on NBC was down 33% compared to the special that aired in prime-time last year for WrestleMania 27.
The one hour special featurin The Rock versus John Cena averaged 2.73 million viewers on Saturday. Last year, the special which showcased The Rock's return to host WrestleMania 27 averaged 3.64 million viewers.
It should be noted WWE was going up against CBS airing pre-season NFL football and ABC airing NASCAR.
This is how the last four years of NBC specials for WrestleMania compare to each other:
8/29/09 air date - WM25 special averaged 3.97 million viewers