NEW YORK—Once the cast of NBC's "Saturday Night Live" was known as the "Not Ready for Prime Time Players." That distinction now seems bitterly ironic for the network.

Last weekend's "Saturday Night Live" with Justin Timberlake had more viewers than anything the network aired in prime time last week. The badly slumping NBC had its lowest prime-time viewership average ever, even counting summer months when TV viewership naturally shrinks.

NBC's most-watched program in prime time was "The Biggest Loser" with 5.9 million people.

Complete "SNL" numbers won't be available until later this week. But based on an overnight sample, the Timberlake show was likely seen by around 8.5 million people.