A search engine that lets users benefit from their social network to improve search results:

delver.com

From the about us page:

What is Delver?

Delver is an intelligent social search engine that enables you to find, experience and benefit from the wealth of information created and referenced by your social world. Our mission is to empower you to easily discover and benefit from the collective wisdom of your social world. Your circle of friends and extended network are increasingly creating and sharing useful information and media online through: blogs, videos, reviews, articles, websites, music… and the list is only growing. By indexing all that shared knowledge, media, opinions, and activities, we can deliver search results that are truly relevant to you.