Silicon Valley lately has seemed like the land of wild—or at least puzzling—valuations.

Facebook bought WhatsApp, a messaging service with paltry revenue and at least a half-dozen sophisticated competitors, for $19 billion. Uber was just valued at $18.2 billion in a round of private-equity financing. Even Beats Electronics, a company with a music service in its infancy and technologically inferior headphones that could fall out of fashion at any moment, was valued at $3.2 billion to Apple.

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