We're working on exactly that issue for our product (Windsoc). We're working on algorithms that attempt to create social circles, while providing an interface that should make manual adjustment and categorization easy. When someone posts using our service, they'll be able to select specific circles of contacts, or even individuals, to Facebook or other services.

I've noticed that people have started to give up on the idea of contact management across services, which is usually a good indication that it's the perfect time to make it happen.

(If anyone is curious, we'll be starting private beta in December.)