The problem with these networks is that they are somewhat elitist and/or restrictive (to the point that many that could benefit from participation in these networks are excluded). Justifications include signal-to-noise ratio, disclosure risks, and/or lack of peer recognition. I was a member of a well-used mailing list for network security types for almost two years until it was decided that I didn't pass the weeding out process (the two times I actually interacted with others from the list involved law enforcement and disclosure restrictions). Two attempts to rejoin the list (required peer "vouching") were only temporarily succussful.
I've since switched jobs but may be qualified to rejoin the list in the near future (Yeah, I'm frustrated by being excluded. I miss the "edge" on various inter-network problems.)