I'm having fun with Ning right now - as the Director of the Social Media Committee of the Web Analytics Association - I made a private, invite only, Social Network for my Social Media Committee, just to make communication and communities easier.

But I noticed that most of the information passing between members (IE: notifications of a new message) is via email - and that bugs me.  Steve Rubel addresses the same issue today in Social Networks as A Place Where Content Finds You:

"… it's basically like having another email account. When people send you messages through the system, you need to log in and use it to read the notes and respond. It's great that these networks are flat hierarchies. However, direct messaging systems add to The Attention Crash.

As social networks begin to really tap into their analytics and search data, they will become virtual brains that know what you're interested in and what you're circle of friends are following. Then these systems will be able to push content and advertising that's highly relevant to you. Further, you will be able to personalize your results. You also will be able to control this setting based on your privacy preferences.

That's how these direct messaging systems should be used - as a funnel for content to find you. "

I think that's what is coming next for Social Networks - but it may take a year or so before we see the first Social Networks that have some of this capability to have content find you based on who you are and what your interested in.