facebook

Facebook | The Facebook Blog - pay attention.  Also note (from their blog) about Facebook search:

"However, we decided that searches carried out on Facebook had a particular structure that only we could understand and optimize for...Some quick stats about Facebook search:

  • Over 600 million searches per month. This makes us one of the top 20 search engines on the web in terms of number of searches.
  • Approximately 1 terabyte of in-memory data.
  • Average search query time of less than 100ms.
  • Most used people search engine on the web.

Facebook search results are sorted by an approximation of social graph distance. People closer to you in the graph—your friends and people in your networks—are likely to be more relevant to you and thus are ranked higher. We also use this concept of "social proximity" to order results within applications like groups and events. Facebook search's key differentiator is that search results are unique to every user because they are based on a individual's place in the social graph."