I wish I could help, but I really wouldn't know. Since Phil works for Ning, he would know a lot more about this than I would. Maybe another fellow Network Creator can shed some light for you.
It seems that the community BLOG feature on a NING network is not accepted as a wordpress, blogger or other blog service by Google BLOG search. This could be the reason why only discussion item, video post and so on are listed in Google - but not the blog feature.