where many people stumble is thinking in one dimension terms, like assuming Adsense is a business model (see recent posts by Josh Koppelman and Jeremy Lui) on just how big you need to be for this to be truly viable.
What I liked in the original article is that it offered a few, less obvious options for monetization.
Consider the following possibilities: your user base turns out to be very broad-based and large, your user base turns out to be smaller, but a significant percentage of them is from a particular demographic or vertical, or your product turns out to be most interesting to other businesses or other websites that want to integrate what you built into their own businesses. Each of these scenarios implies a different approach to monetization.
Given the way your understanding of a market can change as you actually meet the real people who use your site, a change in targeted user base can happen to a lot of different businesses. And if you spend too much time up front working on revenue, you may end up throwing all that work away when you confront the actual opportunities that present themselves.
For some businesses, you may have a precise understanding of who your customers are going to be in advance, and then that turns out to be exactly who they are. If that works out for you, cool. But my guess is that that's not the most common scenario, especially for a website.