Brad Fitzpatrick on the Social Graph problem:

There are an increasing number of new "social applications" as well as traditional application which either require the "social graph" or that could provide better value to users by utilizing information in the social graph. What I mean by "social graph" is a the global mapping of everybody and how they're related, as Wikipedia describes and I talk about in more detail later. Unfortunately, there doesn't exist a single social graph (or even multiple which interoperate) that's comprehensive and decentralized. Rather, there exists hundreds of disperse social graphs, most of dubious quality and many of them walled gardens.

Social graph is becoming an important infrastructure for content/ad/goods distribution. Facebook emerged as an early leader in this space, but, clearly, Google intends to wrestle this control point from them. Brad proposes an open framework that would benefit a major established player, which doesn't have access to the graph.