I have some connections to the venture capital community which does a lot of research on social networks and based on their research and interest - the social aspect of the Internet is the fastest growing segment right now. Even the lawyers are talking about the growing # of social networks because their clients are creating them (there are some interesting legal issues around hosting user content). However, the growth area is now in focused niche social networks.
For example, there is a lot of interest in creating social networks in the health care market.
I assume you are doing this for profit motives, so you should first start with identifying a profitable niche, determining the profile and demographics of your ideal participant and establishing your monetization model (how you are going to make money).
Once you establish a viable business model- the biggest challenge is getting traction and momentum - and this is the challenge of most any Internet business. At this point it will come down to marketing. I suggest you get in touch with the owners of a few large networks on NING and get their thoughts on how they got started. It takes a lot of focused work to get this traction but once you do, it is exponential (meaning the growth will hopefully become viral).