PUBLIC MARKS with tags use & social

RDF is worth learning for a different reason — the profound enlightenment experience you will have when you finally get it. That experience will make you a better format and data API designer for the rest of your days, even if you never actually use RDF itself a lot. (You can get some beginning experience with RDF fairly easily by writing and modifying simple files like FOAF and DOAP for social networks and software projects, or RDFa extensions for XHTML.)

So - what’s the use of FOAF for Social Network Portability?

Is this the silver bullet?