Anyway, yes, that does help clarify. SO I've been wondering about these different experiences of something with a defined boundary (an email group, a web based discussion -- both password protected and publicaly readable)

First of all, on the drift factor and finding other things in common beyond the primary topic. My experience is that this varies wildly. Some groups are so intent about their focus that their practices resist drift and peripheral interactions. Others, mostly those with a more community or social bent, seem to find space for that drift, much of which becomes productive. I think of http://www.shareyourstory.org - which is ostensibly about families with babies in the NICU supporting each other -- and how this whole other layer of daily life, humor, music, food, layers on. Those layers really add to the depth and perhaps breadth of the member relationships. We go beyond the dimension of the lable of "parent of a NICU baby." The same can be said of "meta communities" that hold sub groups of different interests, and the very interesting stuff that happens between those areas - the cross pollinization.

Now, when I think about blog, it is more this "cross polinization pattern" that shows up for me. Even with very focused blogs, there is something "available" through links that expands to wider or diverse topics. It is even easier to cross pollinate, if one has the time and attention to do so. (Same isssue with more traditional online communities - they benefit from those pollinators with time and attention!)

I guess the bottom line is how available and useful are the borders and peripheries. How productive can they be without distracting too much or fracturing attention. Delicate and intersting balance!

I'm rambling. Jeeze, more tea, more tea! No, more chocolate!