Good points all.
The social graph project has roots in something called Friend of a Friend, or FOAF (disclaimer: I worked on a rival project called LOAF, and you may feel free to ascribe everything I say here to purest bitterness).
The idea of FOAF was that everyone would create little XML snippets that represented their interests. For example, if you liked burgers and had a huge crush on your neighbor Matt, you could generate an RDF file that said so and stick it in to your Geocities page.
The problem FOAF ran headlong into was that declaring relationships explicitly is a social act. Documenting my huge crush on Matt in an XML snippet might faithfully reflect the state of the world, but it also broadcasts a strong signal about me to others, and above all to Matt. The essence of a crush is that it’s furtive, so by declaring it in this open (but weirdly passive) way I’ve turned it into something different and now, dammit, I have to go back and edit my FOAF file again.