Overlap of User Communities in Social Networking Sites

As the ability to aggregate your friends on various social networks happens via Open Social,  Compete asks “How do the user communities of the social graph overlap today?”  I was wondering this myself as I'm contemplating whether or not one needs to have a presence on one or more social networks as part of the marketing mix (assuming you've decided that a social network profile is appropriate for your organization).  It begs another question, "Who are the early adopter cross-over social networkers?"

Not sure if you can answer that question, but Read/Write Web gives a summary of Repleaf stats here.