One of the most important lessons we took from delicious was the value of single user utility in social systems. It might seem odd that systems designed to leverage interactions between people can have (should have?) single person utility. But I strongly believe they should.
In short; single user utility is the fact that an application or service can be valuable to a single user with or without the social components like the network, sharing, etc.
Delicious is a good example of this, as Wilson describes, but there are many others. Path, Twitter, Foursquare, Flickr, the now discontinued Nilai, and many more have their own single user utility.
Bad examples? Google+, Instagram. Both of these would be fairly meaningless without the social components.
Social components should multiply the value of a service not be the only value of a service.
As I mentioned last week the new Path application for iPhone is arguably the best designed application on the iPhone currently. And when there is a great design, there are going to be those that begin to pick it apart and do neat things with it. The Path app is no different. People have begun breaking it down in various ways.
One of the distinct features in the Path app is the fly-out menu that allows you to share photos, comments, location, etc. Two developers took it upon themselves to build out that menu both in CSS3 and using CoreAnimation. Both open sourced their work and gave credit.
Path is a private social networking application that allows you to create a path (comments, photos, location, etc.) and share it with a select number of people. Their most recent update is absolutely stunning. Arguably the best looking application on the iPhone at the moment.