It is only logical that Stack Overflow, Server Fault and Super User attract different people, but my guess is that the intersection should be rather large. Yet it seems to me that people's behavior on the three sites is pretty different. For example, I have the impression that the Super User and Server Fault communities don't like to upvote. Neither questions nor answers get any upvotes.
Is this because the number of users differ so greatly? Or because people do indeed exhibit different behaviors? Are votes/user and comments/user significantly different?