BusinessWeek Reporter Ben Elgin provides a nice overview of Yahoo!'s "differentiation strategy" (vs. Google) using community and "social search." (I believe JudysBook has tried to TM the term "social search.")
I also wrote previously here about how Yahoo! is integrating community across its site in Answers, Shopping, Trip Planner, etc.
We'll also be doing an update of our 2004 White Paper "Social Networking: Building a Better Local Online Marketplace" this year.