I think he means that you are signed up on the fan page's mailing list. Companies of which you are a fan gain access to your social inbox, which I would assume

I think he means that you are signed up on the fan page's mailing list. Companies of which you are a fan gain access to your social inbox, which I would assume allows them to garner much more of your attention than if they reached for your spam-filtered, labels-and-filter-ridden, adblocker-protected email inbox. Another intangible but real benefit is the backchannel that fan pages open from the users to the company, and the other fans. Hard to quantify indeed. What I am curious to see is hard data on acquisition cost of fans vs. mailing list subscribers, and subsequent mailing conversion rates.