Dropbox’s ascent has been just as stunning. The 50-million-user figure is up threefold from a year ago, and it has solved the “freemium” riddle, with revenue on track to hit $240 million in 2011 despite the fact that 96% of those users pay nothing. With only 70 staffers, mostly engineers, Dropbox grosses nearly three times more per employee than even the darling of business models, Google. Houston claims it’s already profitable but won’t reveal margins.

It’s only going to get better. That 96% of nonpaying customers is throwing their stuff into Dropbox at such a pace that thousands of people each day blow through the free 2 gigabytes of storage, and upgrade to 50 gigs for $10 a month or 100 gigs for $20. Even if Houston doesn’t sign up a single customer in 2012, his sales will double. As we go over this math Houston pauses to garnish this lovely inevitability: “But we will sign up many, many customers.”