Facebook vs. Google - Network effects

Look at this chart.  Facebook is gaining user eyeball time while Google has stagnated. Is this because of network effects?  Clearly social media becomes more valuable as the number of people you know or want to know on the network increases.  The question is how valuable is this effect?  And to what extent can these types of social network effects be exploited by marketers in building more effective and productive sales channels/eco-systems?

At Comunicato we are working with a number of Fortune companies to explore how various tools including social media can expand and improve the quality of interaction between prospects, customers, sales people and channel partners.  Or in our parlance, how can we configure these tools and techniques in a manner that get more and most importantly, more productive conversations started?