Keep in mind that I wrote that post seven years ago. To respond to your questions:
a) Nothing, except without usage or adoption, no one would use it. In fact, this is what happened with Google’s +1 button (which I worked on).
b) Many do, because parties besides Facebook scrape and use this information. That’s what the “open” in Open Graph Protocol was about. My contention was that Facebook shouldn’t have invented a new [invisible] metadata format but merely relied on the visible content on the page. In hindsight, my goals were different from Facebook’s or publishers.
c) Many do.
The thing I was pointing out was about the contrast between Google’s unauthenticated approach to PageRank relative to Facebook’s own page graph that would have been enhanced by knowing what users actually like and then being able to recommend or surface content based on what friends of friends liked. Social media ending up probably doing more for social discovery of content then social search ever did, so in some ways, the future I was contemplating in this post never came to fruition.