The startup had millions of users, but, from the beginning, just one customer.

The predominant way of interpreting Facebook’s billion-dollar purchase of Instagram, in light of the social-networking giant's forthcoming IPO, is that Mark Zuckerberg had to pick up the photo-sharing app to boost his company’s mobile engagement. That would allow him to guard the mobile flank against incursions from Google, Twitter, and whatever other social-media tools might next arise.