I agree with many of the points brought up. It's strange to me that Facebook would consider a practice that limits the number of eyeballs on their pages. For a social network, the real competitive advantage is the number of users and how engaged they are. While they may make more money in the short term, cutting into the developer ecosystem (healthy as it is now) will cut into their reputation and eventually their bottom line.
On the other hand, I could see them charging just the top 1% of the heaviest API users. In that case, it would fit Michael's cases for when charging for an API is ok.
True, their user data is their product, but the Forbes article wasn't talking about charging for Facebook Graph API access, it was talking about charging for Facebook-as-SSO-provider. Facebook Connect is the means by which Facebook can mine even more data about their users which allows them to increase the value of their product.
Facebook Connect grants your site access to more of Facebook's 'product'. Regardless, in my opinion An SSO/Identity Service seems like a reasonable API to charge for. Developers won't go elsewhere until the products, err users go elsewhere first. Facebook's value is always going to be about data and who has access to it.
On the other hand, I could see them charging just the top 1% of the heaviest API users. In that case, it would fit Michael's cases for when charging for an API is ok.