Joshua Porter writes in “Why Invest in Social Features for Your Web Site?“:
In addition to the explicit benefits for the site owner, implementing social features means building a community around shared experiences. The notion of Ã¢â‚¬Å“shared experiencesÃ¢â‚¬Â difficult to define, but the benefits of increased participation and caring are clear. People respond best to communities where they believe theyÃ¢â‚¬â„¢ll find like-minded people and where they feel their ideas and opinions matter. This trust is the real benefit of social software.
Therefore, adding social features isnÃ¢â‚¬â„¢t so much a leap of faith as it is an investment in a long-term experience design strategy.
It’s a great article, describing some of the softer or harder to quantify benefits of adding social features to your site, and connecting them to core user experience design issues.