Why is Instagram worth $1 billion to Facebook and Zuckerberg?

“Why in God’s name is Instagram worth a billion dollars to Mark Zuckerberg?”

I gave it another 45 minutes’ worth of thought. Troublingly, my best theory was that Mark Zuckerberg stood to inherit a trillion dollars from his eccentric uncle, but only if he could spend a billion dollars in less than an hour without acquiring any tangible property.