Preliminary Nielsen ratings on Thursday showed that an average 8.5 million Americans - about 3.5 million down from the 2011 opener - watched Spears and singer Demi Lovato make their debuts in the two-hour season premiere of "X Factor" on Wednesday.

But on NBC some 11 million tuned into the finale of "America's Got Talent" and 10.7 million watched a third episode this week of "The Voice."

NBC emerged as the most-watched U.S. network of Wednesday night but Fox was the winner in the 18-49 age group most prized by advertisers.

"The Voice," the brightest light in struggling NBC's programming for several years, was moved this year to two seasons a year in a closely-watched decision that put the singing contest compete for viewers with British entrepreneur Simon Cowell's revamped "The X Factor" on Fox.

Spears, the biggest pop phenomenon of the 2000s with hits like "Toxic" and "Womanizer," was recruited by Cowell earlier this year after he fired two "X Factor" judges following a disappointing first season. Spears, 30, is reportedly being paid around $15 million a year.

But Spears and former Disney Channel star Lovato were a hit on social media. Social TV analytics firm Bluefin Labs said that "X Factor" triggered about 1.4 million comments on Twitter, Facebook and other social media platforms on Wednesday, making it the most social season premier of all time.

Fox is a unit of News Corp and NBC is majority-owned by Comcast.

(Reporting by Jill Serjeant; Editing by Marguerita Choy)

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