Facebook mania could be putting a crimp in investor perceptions of News Corp. (NWS), the parent of its chief rival in the social networking sector, mighty MySpace.

In a detailed report today, Bear Stearns analyst Spencer Wang takes a close look at how the recent surge in usage of Facebook might be affecting MySpace - the No.1 traffic in the U.S. in terms of page views - and what it might mean for investors in News Corp. “Facebook is emerging as a serious competitor to Myspace,” he says. He notes that Facebook’s user base has grown 46% in 5 months and should reach 50 million users by the end of this year, and 100 million by mid-2008.

“Myspace will need to respond,” he says, noting that the number of minutes per user for MySpace has flattened. Wang notes that the number of unique visitors and the number of page views among users aged 12-17 fell about 25% in the June quarter versus last year. “The data may suggest that Facebook is capturing more mindshare among this leading edge demo,” he writes.

Wang says he doesn’t see any near-term revenue risk for Myspace, given low but improving monetization. “The risk,” he says “is that the growing traction for Facebook could negatively impact sentiment on Myspace/NWS.”

At first glance, that seems a little silly, since, as Wang notes, MySpace in 2008 will be just 3% of the company’s revenue, and 5% of earnings before interest and taxes. But he also notes that News Corp. shares are up 41% since the company bought MySpace, beating the 26% rise in the S&P 500 over the same period. Wang contends that “the bulk of the stock performance can be attributed to MySpace, as financial guidance for News Corp. has been relatively constant over this time period.”

Wang contends News Corp. is going to have to take steps to respond to Facebook’s more open approach, or face increasing erosion of the MySpace user base. Writes Wang: “We think there are some early signs that ‘engagement’ of MySpace users is peaking out, reinforcing the need for more innovation.

Or to look at it another way, Wang asks: “Is Facebook to MySpace as Google (GOOG) was to Yahoo (YHOO)?”

News Corp. today is down 24 cents at $23.78.

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