Daniel, you make a very good point. The conversation is where the sale happens, not the post itself. I think it's tough for marketers (myself included) to think of blogs as a completely separate medium than a business website. We're so business focused that we forget that a blog is a place to share opinions and information with other people, and not push a hard sale.
A common marketing metric around blogs is how many people the blog pushed through to the website, or how many leads it generated. But a blog and other social media initiatives should be about sharing information, building awareness, and partaking in the conversation that comes about from the content you or others develop.
From a business standpoint, it's hard to put solid ROI around these figures. How much is your reputation worth? How can we track the people who we talked to on the blog or other SM channels and then find out whether they bought our services? It's not incredibly easy, but it's also not impossible.