Given the really low barrier to entry it seems like this is very low number. I'd say Ning hasn't reached its product/market fit yet :-)
I created a private network for my family a while back and thought I'd invite everyone but I got frustrated with the post-setup and gave it up. I probably should probably try again and send in some feedback.
I do think 70% active after 30 days seems good. I wonder what the drop-off is at 60 and 90 days though.
The numbers would be higher for 60 and 90 days. The statistic is that 70% of networks that were ever created have been used within the last 30 days before today. The other 30% were most recently used more than 30 days ago.
I think the only one i have heard of was a playboy social network that got like 30 users in 6 months and was featured on TechCrunch - (and still only had 30 users)