What’s really behind Microsoft’s $44.6 billion hostile bid for Yahoo? If you believe the tech giant, people — not market share, advertising deals or products — are its main motivation. “People are the single most important asset in this combination,” Kevin Johnson, president of Microsoft’s platforms and services division, told Microsoft (MSFT) employees in an e-mail last Friday.

So which key executives should Microsoft try to keep as it moves to oust Yahoo’s (YHOO) board? We asked people close to both companies, including investors and current and former employees, who Yahoo’s most indispensable Yahoos are.

• Jeff Weiner, executive vice president, network division

Yahoo lags when it comes to search ads, but it’s still ahead of Google on one count: Traffic. According to the most recent comScore data, Yahoo sites are the most popular, with 138 million visitors a month (Google has about 135 million). As head of Yahoo’s consumer products — Yahoo Mail and Messenger, and the company’s finance and news sites — Weiner oversees Yahoo’s “front door,” a valuable asset to Microsoft. Under Weiner, Yahoo recently reopened its main site to third-party developers in hopes of expanding its reach even further. The rising star, hired on by former CEO Terry Semel, also led Yahoo’s efforts to revamp its online ad system, called Panama, before taking on his current post.

• Caterina Fake, co-founder of Flickr and head of Yahoo’s technology development group

Let’s face it, Microsoft isn’t exactly known for being hip. Its lack of social media-savvy in particular is why sources say Fake, who started a hit photo-sharing site (Flickr, which Yahoo bought in 2005) under her belt, could be an asset for Redmond. As head of Yahoo’s technology development group, Fake is now in charge of finding the next Flickr and would bring her deep understanding of online communities to Microsoft.

• David Filo, Yahoo co-founder

Yahoo employees say Filo is both well-liked and respected within the company. Filo acted as interim CTO last year before Aristotle Balogh was hired for the job. His value to Microsoft is his longstanding relationship with Yahoo’s engineers — which Microsoft needs. “I think Filo is really not given enough credit for how much the engineers love him,” says Eric Jackson, a Yahoo activist shareholder. That said, keeping a billionaire founder around won’t be easy for Microsoft.

• Usama Fayyad, chief data officer

Sure, an acquisition will give Microsoft access to Yahoo’s extensive user database. But if they need someone to make sense of those numbers, then they’ll want to keep Fayyad, a former rocket scientist who now oversees Yahoo’s data strategy.  Data mining is also key to using behavioral targeting in advertising. Fayyad is no stranger to Microsoft though — he did a five-year stint leading the company’s data mining research group way before coming to Yahoo. But it’s not clear whether he’s eager to work for the software giant again.

• Susan Decker, president

Since joining Yahoo in 2000, Decker, a former Wall Street analyst, has earned respect for her candor. She’s also credited with transforming Yahoo from a seller of advertising on its own site to an ad player across the web and driving Yahoo’s acquisition of Right Media, an online marketplace for advertisers. Assuming Microsoft could persuade her to stay, Greg Sterling of technology research firm Sterling Market Intelligence, says it would be a good move for the Redmond-based company to retain Decker.