"We believe social search is any search aided by a social interaction or a social connection… Social search happens every day. When you ask a friend “what movies are good to go see?” or “where should we go to dinner?”, you are doing a verbal social search. You’re trying to leverage that social connection to try and get a piece of information that would be better than what you’d come up with on your own.
We know that because of the volume of searches like this that happen everyday, that the social component of search is actually very important, and it hasn’t translated well yet to the internet medium.
Social search is hard because the intuitive thing you would do online to mirror normal social networks and other social interactions just aren’t that effective, compelling or even reasonable. So, for example, from the Facebook News Feed analogy, you could just get a social network and broadcast all of everyone’s searches to everyone on their social network, but most people view search as a far more private activity than that. They’re not comfortable letting everyone in their social network know what they searched for, so such a product is clearly not reasonable."
This comes from an extensive and must-read Q&A with Marissa that was published yesterday on Venturebeat, at roughly the same time as Google released their awaited Social Graph API. Techcrunch comments on the API too.