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Google Plus is Google’s latest attempt at building an explicit social graph that they control, but Google has been building out an implicit social graph for quite some time …

This all changes with Google Plus. One of the limitations of building an implicit social graph is that you don’t have the data to test against to confirm the predictions and relationships that graph discovers. It still has to depend on the data made public, but is limited by relationships that are held private (aka Facebook). Google Plus, among other things, creates a massive set of explicit social graph data, which can be used for machine learning and accuracy checking.

via Building The Implicit Social Graph.