I'd be interested in an analysis of what it is about communities that makes them more or less attractive to men vs. women. There must be some moment in the creation or burgeoning of a community when it could really go either way - predominantly female, predominantly male, mostly equal - and then afterwards, it seems self-perpetuating (e.g. reddit and pinterest).
But what is it that tips it one way or another? It seems stereotypical, but are sharing/socializing, versus debate-oriented, communities more likely to be welcoming to women versus men? Is it a female preference? Along that same line of thought, is the issue instead that men shy away from communities that are predominantly about positive reinforcement, sharing and socializing?