I've thought about this very application (I call it "CRM for hypersocial people") and the most important component is the natural language processing. In particular, you want good autotagging and query expansion, with an emphasis on recall.
More specifically, if one of your ballet dancer friends is manually tagged "ballet" but not "dance", and you search for "dance", you want this friend to appear in the results.
Additionally, if you are looking for "hackers", it should know that it should include people who are "hardcore programmers" or maybe those that program "ruby".