Wow, Russell, this is really interesting. I'd love to see other customers jump into the thread or your blog post with their thoughts or stats.
One line of questioning that comes to mind is generally how is your social media intranet site being used? Are people referencing content from other enterprise systems? Is it mostly self-contained content?
Interesting question. I started looking at the time spent on a page numbers. Assumption is that community pages have interesting content that people want to know. Looking at the number of visits and the amount of time spent gives an indication of how interestng/important the information is, after adjusting for the amount of content on the page.
As you indicated, the purpose is important. If this is a one-stop with links to other information outside of the community, then a high bounce rate is OK.
Once again, we need to understand what the number means in each context and not create pretty charts jsut because we have numbers.