Ronald Burt pointed out the importance of "structural holes" -- those nodes (people) that connect networks. If I know person A and person B and person A and B ought to know each other, but don't, I am occupying a "structural hole" in their intersecting social networks, and making that introduction could create social capital for me as well as them. Substitute "idea" for "person." This is where I live and why I hang out online for no really well-defined purpose. Burt's paper is a 58 page PDF.