In his February 2009 webinar on user-submitted questions, Google software engineer Matt Cutts discusses whether or not Google has changed the relevancy it awards to social media sites.
Has Google changed the relevancy it awards to social media? –

In a talk that centered largely on background information about how and why Google does things, Matt makes the observation that Google is constantly tweaking its algorithms and makes somewhere on the order of 300 to 400 changes annually. No doubt some of those changes have affected how Google scores social media sites, he admits, but declares that this is an incidental outcome of the constant process of refinement rather than any dedicated effort to treat these sites any differently from any other.

He emphasizes that Google is concerned with gross value for the end user, rather than getting wrapped up in categorizing websites in terms of cultural or philosophical relevancy. Put another way, the idea is to give high rankings to those sites which will answer the question posed to the search engine, whatever the question may be, rather than getting into the issue of whether it is a worthy question or not.

To summarize his position, Matt states categorically that Google does not adjust the relevancy of social media sites in any way as a result of their being social media sites. No dedicated project for such categorization is underway or contemplated at the time the interview was given.