You can turn eyeballs into money, but it helps if they're the right eyeballs. The less discriminate you are in who you target, the less you understand your audience, and the less money you'll make per visitor. For some websites, this value will approach zero.

Knowing specific people who will use your product is important, if only to better understand your own value proposition. To rely on people discovering your site and immediately understanding what it offers them is silly. This is true regardless of your business model.