I think you are mistaking how to integrate the social graph into search. You don't do it by exclusively searching within the social graph, you use the social graph to augment your traditional search. The best way for me to show you what I mean is for you to look at the attempts blekko is making. Create an account, and connect your facebook profile, wait 1 hour, and then do searches with "/likes" appended. You'll still get mostly normal results but the instances where a friend has liked a site will be featured more prominently. Facebook has not been able to "just make a search engine" with the social graph because making a traditional search engine is a non-trivial challenge.