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.