I don't think he's claiming the variation in content has decreased, but warns that dependence on collective rating systems could decrease the variance of an individual's effective sample size.
I also don't think the author claims the web has introduced any social problems, just that it isn't solving them yet. Are these even problems with technical solutions?
I think I understand Jaron's premise - sites like facebook reduce the human element by forcing its users to add information according to a predefined structure. Presumably these same users may have created more free form web sites (and he equates this to individual creativity), but I don't really agree that it leads to group-think.
Facebook or any other site is just a medium - whether or not it leads to group-think is more a function of the participants than any conformity in their online 'meeting place'.
