UPDATED: Brian Williams’ new primetime newsmagazine Rock Center appears not to be the answer to NBC’s ratings woes. Halloween played a ratings trick on the new show, which opened with a low 1.0/3 in 18-49 and 4.1 million viewers at 10 PM last night. That is below the average for the canceled The Playboy Club that Rock Center replaced (down 38% in the demo from The Playboy Club premiere last month) and lower than the performance of NBC’s veteran newsmagazine Dateline on the less-trafficked Sunday night. It marked NBC’s lowest Monday debut ever. Rock Center‘s lead-in, The Sing-Off (1.4/4), was down 7% from last week to log its lowest rating ever. NBC (1.2/3, 4.3 million) once again finished fifth on Monday among adults 18-49 behind the other major English-language broadcast networks and Univision.

Halloween probably wasn’t the best night to launch a new series as TV usage was low because of the holiday, though the 10 PM hour was not as impacted as 8-10 PM (down 10% vs. last week and 8% vs. last year.) As can be expected, the 8 PM hour, when trick-or-treating is still in full swing, was hit the hardest, off 13% from last week and 11% vs. last year. As a result, all networks were down week-to-week. That is despite possible inflation to the ABC and CBS fast-national ratings because their stations in San Diego and Kansas City, respectively, carried the NFL football game. Fox’s Terra Nova (2.1/6) was down 25% from its last original two weeks ago, and House (2.5/6) was down 19%.

CBS’ How I Met Your Mother (4.1/13) was down 5%, 2 Broke Girls (4.2/12) was down 7%, Two And A Half Men (4.7/12) was down 15% to a season low, Mike & Molly (3.8/9) was down 10% and Hawaii Five-0 (2.9/8) was down 6%. CBS (3.8/10, 11.3 million) won the night in 18-49.

ABC aired a rerun of the Scared Shrekless special (1.7/6, down 35% from last year’s premiere on NBC), followed by Dancing With The Stars (2.8/7), which was down 13% from last week to an all-time low for a performance show. At 10 PM, Castle (2.9/7) bucked the trend, up 21% from last week. The mystery series always gets adjusted down because of Dancing overrun but looks on track to post a net ratings gain. The network (2.7/7, 13.9 million) finished first in viewers. The CW aired back-to-back Ringer repeats. ESPN’s Monday Night Football also felt the impact of Halloween as the San Diego Changers-Kansas City Chiefs game drew a 8.7 metered market average, below MNF‘s average season-to-date (9.5).