A different take on social networking  
Yesterday I came across a startup that offers a different take on social networks. AFAICT it’s biggest differentiation is the ability to easily create an affinity group. And individuals can easily become part of or leave a group. So whereas Facebook defines a single relationship, Sparkcloud lets you participate in multiple relationships. What I would like to see added is the ability to selectively share information with each group so that a professional interest group need not see your personal info and vice versa.
Check it out for yourself: http://www.sparkcloud.net