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When it rains, it pours. And eBay's recent billion-dollar writeoff of Skype, the VOIP startup it bought two years ago, could have an impact on Facebook's negotiations to sell a stake in the social network, at a high valuation, to Microsoft or another large backer. (Both Bernhard Warner and Kara Swisher make this observation, which I'll attribute to great minds thinking alike.) Skype's financial failure is a sobering reminder of the risks of overpaying for a startup. And all of a sudden, Microsoft CEO Steve Ballmer is playing diffident, saying Facebook "might be a fad." But what may be forgotten in this latest skeptical turn to the hype cycle is that underpaying has risks, too.

No one is more familiar with the pains of a lowball bid than eBay itself. It tried, repeatedly, to buy PayPal before that startup went public, finally buying it for $1.5 billion after its IPO. In retrospect, that struck PayPal's founders and investors as low, but it was a good price in the depths of the dotcom depression earlier this decade. And most importantly, eBay could have had it even cheaper if it had just bid a bit earlier. That experience surely lingered in Meg Whitman's mind as she negotiated the Skype purchase.

Microsoft, likewise, can look at Yahoo's experience, missing out on chances to buy Google, YouTube, and even Facebook on the cheap. Yahoo today is surely not the better off for having failed to clinch those deals.

So yes, Skype's a good reminder of the risks on the downside. But it would be foolish to think that Microsoft's balance sheet, bursting full of cash, couldn't handle a $500 million writeoff. Or more. The real question: Can Microsoft's executives handle the marketplace's perception of the company's irrelevance, a perception that grows relentlessly with each passing day?