We must make Ning great for managing content. Ning offers more features than many custom websites for doing it, and Ning is free.
My network activity is on a niche (culture viewed through art auction). This is an intellectual game invented by myself. My choice was to play it through a fully open Ning network. Fully open excepted that I do not offer members the capability to create Groups, because I wish a full control of the architecture of the groups.
I strongly recommend my method to fill the content : I endeavour to publish a daily post. Six months mean 180 posts. The content grows to high figures through an effort that is regularly spent throughout the year. Within monthes I will be able to prepare syntheses that will be totally based on my data, and these syntheses will be worth to be presented to professionals.
In the mean time, everything is open to visitors. They can view, link, copy whatever they wish from my content without being commited to join. Those who join are welcomed but I know that some regular readers did not join. So I do not care at all having 10, 100, 1000 or 10000 members. These figures do not mean anything if among them there are 9, 99, 999, 9999 sleeping members.
in my opinion, the prerequisite for managing content on Ning is to have a goal, a plan, and a strategy to apply it. Memberships and friendships are tools, not goals.
Use Ning for spreading your content. One of the futures of the web is possibly to re-enable cultural awakeness and scientific teaching through the networks.