Just so ya know...
Network owners "should be" able to use their "Network Places" on their computers to access their networks[s] on Ning by dialing into it. [just type in the address] Normally, you'd sign in with your Ning Network info, then just grab all the files, just like when you access your c-panel on a normal website space, and access the file section.
IF, in doing that, you find it won't let you in, request a "support order" through Ning to turn on your access to do so, that you wish to back up your files [some will already have this from having added things to their networks]. Whether or not they come up with a good migration system, this way works, then you can upload it anywhere else, or have as a back up.
Speaking of migrations, Be Careful....in my experiences with such things, once you choose to migrate from such a network, it takes all the files, and deletes them altogether from the first platform, so it's not like you can do this more than once or to multiple other networks, unless they have changed that recently.