Making your social network the place to be

Now that there are more social networks than butchers shops (I made this up but no doubt true) what makes one more important than the other?

It's pretty much common thought that business networks will grow very quickly now that the general usage of the likes of Facebook and Bebo are the norm.  But Facebook retains a high level of usage due to the high number of members; I even found a long lost buddy living in the back end of Oz via Facebook. 

But what makes a new network worthy?  And then when it gets really big, how does it retain its value?  For example, a site that carries a lot of information will in the early days be easy to follow but as more and more members add information, the more work the member has to do to find what they need more easily.  So does a niche business site remain good by staying small or is it about simple navigation?  It's all work in progress finding out what makes good, a lot of which is defined by members, but getting it right will hopefully pay dividends.

So many questions, so many thoughts.  I may need another holiday quite soon.