Empower people to create their own social sites. I played with the beta a while back and it seemed to be decent. I believe you can download your source code from their hosted service and deploy it elsewhere.
They are showing that it lets you store your own pictures, video, do your own blog, etc.
Ning has full RSS and a REST api. You can reprogram it or add features to it. You can hit the view source button at anytime and then customize the code.
They seem to be betting that the ability to customize the social network will appeal to users.