What are mutual friends on Facebook?

What kind of data and algorithms could calculate a "mutual friends you didn't know you had" feature for Facebook?

• So there are lots of cases where I have a Facebook friend whom I have friends with because we are members of a larger circle of friends, e.g. we are coworkers, or we went to camp together.  In those cases, we will often have a horde of mutual friends (e.g., our fellow coworkers or campmates) so numerous that it is difficult to browse through. But sometimes there are unexpected mutual friends!  And those are interesting cases to discover.  If you were to build an "unexpected mutual friends" feature, how would you go about doing it?

• Answer:

  I've thought a little bit about this before.  My friends network tends to have certain strong clusters: high school friends, college friends, coworkers from different companies, etc.  It would be interesting to run a cluster detection algorithm, and then find all the friends of friends who are connected to multiple clusters.  I think that's along the lines of what you're getting at.