I'm not extrapolating that every closed group is optimal. Hubski's follow mechanic doesn't simply create a closed group, it creates my own personal closed group that I have complete power over. It is very easy for me to instantly reshape the group to fit my personal definition of "optimal". You say that this has the potential to destroy diversity of opinion, but that's up to to the group maintainer. If someone excludes people who have opinions they don't agree with, that's their own fault, and it's done because they decided that's how to make their group fit their own personal definition of "optimal". Not every person will want their group to be that way.Now a private group of people that all know each other can be great in spite of being closed. But it's incorrect IMO to extrapolate that all closed groups are close to optimal.