Hello Mark,
Thanks for the comment.
What you said is true. Many communities can have social network integrations (as you mentioned with Yelp). In fact our own community platform is also developing Twitter and Facebook integration to enable social networking capabilities in communities. But social network services are also developing community-like interactions, like Groups in FB and LinkedIn. There is a social anthropological reason for all these trends. And this is a topic that I will cover in the next few post -- to explore the dynamic interplay between communities and social network. It is not accidental that communities and social networks are the two most robust social structure in human history. They needs the other and complement each other.
I will address this issue in greater depth in later posts. Hope this will temporarily address your comment for now. I will have to ask you to come back for the full blown treatise on this topic.
Hope to see you back next time.