> If I was Google I would be providing ... a loose social network that encouraged links to other sites (perhaps similar to HN on a larger scale) that would be easy for third-parties to integrate.
They did. It was called OpenSocial, which originally released in late 2007, and was implemented by MySpace and others to compete against the Facebook platform (also released in 2007). I remember when I first looked at its complex XML documentation, that it was much easier just to use the Facebook API.