I just took a better look at Open Graph. Certainly marking up pages with RDFa tags using standard Ontology's for properties and classes could be a useful thing.

The problem as I see it is that the Facebook corporation strongly discourages spidering the Facebook web site. It is certainly their right to do so, but from a research point of view it is a shame that it is not allowed to spider a portion of Facebook to get the RDF graph. It is difficult to know if something is useful if you can't play/experiment with it.