This is a reprint from last year but well worth your read if you missed it. The ranking for the search engine is imperative.
Excerpt: You can be certain Google is building an implicit social graph:
“we studied the implicit social graph, a social network that is constructed by the interactions between users and their groups. We proposed an interaction-based metric for computing the relative importance of the contacts and groups in a user’s egocentric network, that takes into account the recency, frequency, and direction of interactions” – Google
Read full article via Building The Implicit Social Graph | SEOmoz.