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. [...] Declaring connections is about as much fun as trying to whittle people from a guest list, with the added stress that social networking is too new for us to have shared social conventions around it."