Facebook doesn't give out granular numbers about Instagram either, but the company's overall success has the Street willing to accommodate Zuckerberg's ambitions, up to a point.
Several analysts wrote approvingly last week about Facebook's decisions to slightly reduce its guidance for how fast it will boost spending this year, Bhatia said, because it gave them a clearer sense that management has a plan and isn't just tossing money around. He said it will take two to three years before the success or failure of this year's investment surge becomes clear.
"The difference between 65 percent and 70 percent is just how fast they can hire,'' Bhatia said. "But they're addressing a $500 billion market, with 1.3 billion people on Facebook and 700 million on [Facebook-owned instant-messaging service] WhatsApp, and they need infrastructure to support all that.''
The worry is that Zuckerberg, 30, and his team won't be demanding enough in deciding which projects are promising enough to fund, or tough enough on fat elsewhere in the budget instead of pumping fresh money into hiring engineers, Spencer said (his fund doesn't own Facebook). He thinks Yahoo will eventually make small acquisitions to try to build a new core, but without dramatic deals.
"I'm more convinced that Yahoo's move is correct, while Facebook's is what it is,'' he said, pointing to operating profit at Facebook that narrowed by 14 percentage points in the fourth quarter, measured under formal accounting principles rather than the pro forma numbers analysts most often use. "Usually with companies of this size and scale, if their margins start going down, it's not a good sign. I've been wrong so far, but this is just getting started."
But Facebook may have learned the opposite lesson from its slow start on mobile, which caused the stock to plunge in the months after its 2012 IPO, Mahaney said. Back then, Facebook was knocked for spending too little to make sure it hit the next big market—a misstep he said shoved both eBay and, yes, Yahoo from their places among tech's elite.