AOL’s agreement on Thursday to acquire social network Bebo for $850 million raises questions about the $15 billion valuation assigned Facebook when Microsoft took a stake.

Based on Bebo's 2007 financials, AOL paid 42.5 times revenue, according to D: All Things Digital's Kara Swisher, compared with the 50 times revenue determined by Microsoft's stake in Facebook last year.

It's a reasonable "conclusion to draw that Facebook got a very high valuation" based on AOL's deal for Bebo, Sterling Market Intelligence analyst Greg Sterling said.

Considered another way, AOL would spend about $21 per member for Bebo's 40 million worldwide users, versus Facebook's $204 per member, based on Microsoft's $240 million investment when the site had 73.5 million users. That's also less than the $26 per member News Corp. paid to acquire No. 1 social network MySpace.

San Francisco, California-based Bebo offers video- and music-sharing features similar to MySpace but lacks many of the widget applications that clutter Facebook.

Bebo’s U.S. audience has nearly doubled since last year, topping 2.1 million unique viewers in February, and its worldwide numbers increased 60 percent from the same period last year, reaching 7.1 million.

“Bebo is the perfect complement to AOL’s personal communications network and puts us in a leading position in social media,” AOL CEO Randy Falco said in a statement.

That remains to be seen. Analysts point to the difficulties social networks have so far had in wringing advertising dollars from their vast audiences.

“The problems faced by MySpace and Facebook in generating advertising revenue were likely reflected in Bebo's valuation,” said Jason Forbes, vice president in the media & entertainment group of consultancy CapGemini.

AOL’s acquisition might be cheap in comparison with Microsoft’s or News Corps.’ but given the challenges MySpace and Facebook have had making money, AOL’s move might still prove pricey.