A VC: Twitter's Business Model.
I agree with Fred on this issue. It's not that we don't care about the business model or aren't thinking about it, but, as Fred says: "But every ounce of time, energy, money, and brainpower you spend on thinking about how to monetize will take you away from the goal of getting to scale. Because if you don't get to scale, you don't have a business anyway."
Addendum: Jason's take: "An absolute idiot with 10-20M users can make a ton of money. So, get to tens of millions of users and forget about money."