Once you have your Ning Membership you have one login for all social networks on Ning. Any Ning member can create a social network. Your social network is like a socially-enabled website. You can add features or modules by dragging and dropping.
Your social network has its own URL - like http://practicalbusinessideas.ning.com/. And once created you can add content and invite members.
Whenever your members login to Ning they will see all the activity on the networks to which they belong. I think it is important to reiterate that there is one login across all Ning social networks - this to me is one of the biggest strengths. Your network is within a network of networks and so you will benefit from the interest created by other networks - because every time somebody logs in to see another network, they will see the activity on your network. So make it interesting!!
Of course, you can keep them coming back by broadcasting messages to them.
Ning Network Management
A further benefit of Ning is the ability to embed Ning media players in your website. You can learn more about Ning at the Ning Product page.
What's in it for Ning? You guessed - Advertising revenue. If you don't want the ads, you pay for the premium service. There are better social media platforms than Ning in terms of features and the premium service at Ning does not close the gap dramatically. However, it is almost certainly a good idea to join Ning even if your primary online community is elsewhere (you could even use it to recruit!).