On the question of who knows best, I'd hope the social interaction designer.
I'm reminded of a story about my Geology professor in Seattle, Washington. I was taking several geology classes at night for fun with a number of other older students, and one day we were out on a field trip. The Professor picked up a rock and identified the rock. While he wandered off to look at an outcrop. His students were left to ponder the question; "how does he know what he knows?"
I suspect that after a while knowledge about communities and social media becomes second nature, you almost run on auto pilot in the same way you know where to turn on your daily commute. In the community you know what to say and what not to say, but sometimes you have to prepare if you are going to experience something new.
Interestingly, I had asked my Geology Professor a few days later how he knew how to identify the rock, he had explained he'd earlier looked at a geology map, when planning the field trip and knew that there would be only certain types of rocks in the area of the field trip. When looking at a rock at random, he had already eliminated most of the alternatives based on that map knowledge. I studied rocks in Seattle for a few years, and after a while I remembered consistently the secret to rock identification was looking at what minerals made up the rock; be they mica, feldspar, or quartz.
To me the older, or more experienced someone is with a community, the more that person is able to recall patterns of recognition. Or as Adrian does, he uses his existing layers of knowledge and frameworks to explain new social media communities he observes.
Adrian, you ask a good question about who knows best. I would have thought the person who is the keenest observer. Though I thought you also made a compelling argument in our conversation about users of a social media community. You said that part of the success of an individual within a community comes from that individual's membership of the group. Without the membership, even the best observer will not have the same credibility. Perhaps we really have two questions here, 1) Who knows best? and 2) Who can participate the most in the community? One person may do both well if they are member of a social group, but it’s unlikely that an observer who is not a member of a group will be able to interact as well as group members.