Maybe audiences don't love

Big Brother

after all.

On Thursday, the new reality show on

CBS

, a unit of

Viacom

(VIA) - Get Report

, saw its ratings slide a precipitous 39% from its

debut on Wednesday, according to

Nielsen Media Research's

preliminary ratings in major markets.

A big factor in the drop is that

Big Brother

on Wednesday, when some 22.5 million viewers tuned in to the show, followed

Survivor

, another reality show on CBS that is already a bona fide hit. Having a wildly successful lead-in, especially during the summer, when the schedule is littered with repeat telecasts, can artificially inflate a show's overall rating success.

Big Brother's

Thursday numbers seem to confirm that. The show drew about 13.5 million viewers on Thursday.

"Losing more than one-third of your audience from one night to the next is eye opening," said Marc Berman, who closely tracks television ratings for trade publication

Mediaweek.com

. Berman predicts that

Big Brother

, which is airing five nights a week, could see its ratings decline to around 9 million viewers by Saturday's episode.

For its time slot Thursday -- 8 to 8:30 p.m. EDT

-- Big Brother

, which throws 10 strangers together in a house full of cameras, placed second to a repeat of

General Electric

(GE) - Get Report

unit

NBC's

longtime hit

Friends

.

Other observers remain more sanguine about

Big Brother's

prospects. The show's ratings are "almost exactly what we thought it would do," said Stacy Koerner, vice president of broadcast research at media buying firm

TN Media

. "That's pretty good for CBS on a Thursday night in the summer."

Koerner said she expects the show to gather more momentum going forward. "I would expect that number to get a bit better as people get to know the people in the house and they start fighting and stuff," Koerner said.

The show's demographics, which will be released later Friday, could ultimately determine its success. Like

Survivor

,

Big Brother

on Wednesday brought in reams of the younger viewers coveted by advertisers, a viewership that has eluded CBS for years. Koerner said there's no reason to think that Thursday night's telecast should do just as well among 18- to 34-year-old viewers. That said,

Friends

has historically attracted large chunks of that demographic.

Walt Disney

(DIS) - Get Report

unit

ABC

, meanwhile, saw its smash-hit game show

Who Wants to Be a Millionaire

win the night, attracting around 22 million viewers, with its first new episode in more than two weeks. ABC won the overall ratings battle Thursday night, while NBC placed second and CBS third.