Could Facebook, a popular Web site where people track down old friends and make new ones, really be worth $6 billion? 

That’s more than Murdoch is paying to snag the Wall Street Journal, Dow Jones newswire, Barron’s and a host of other assets.

But that’s the rough valuation Bear Stearns comes up with in a research report that makes the case for Internet firm Yahoo! to invest in social networking sites. 

“Our opinion is that Yahoo! needs to step up its initiatives in this area (through acquisition or partnership) relatively shortly, or risk being marginalized in some of their businesses,” the note, by analyst Robert Peck, reads.

“Further, while valuations could look daunting at first blush, we believe the rapid growth and engagement of these sites provide strong monetization opportunities… Our rough analysis indicates Facebook could be worth as much as $5-6 billion”,  it reads.

The report continues: “We believe price could have been the major deterrent to Yahoo!’s past pursuit in the space. We believe Facebook, Bebo, Friendster and MySpace are all sites that Yahoo! has considered, and when it comes down to it, we think the price tag is the ultimate factor for a transaction to materialize.”

Of course, this is all theoretical unless Facebook decides to sell - and that’s a big if.