Talking through the current success of Facebook and the defection of users from MySpace, it occurred to me that the way users search on both sites has a lot to do with the changing of the guard. So here's a functionality view rather than a strategy view.
Facebook's structured search, with the ability to find different types of pages based on
2) form-field searching is designed to find and identify specific people
3) search results grouped by content type
When you type a name of a friend or a Page, you get an 'auto-complete' which gives you a list to choose from of all your relationships. This is a very structured search, pulling real (and guaranteed) results from the database. Having a real ID in Facebook helps this immensely - first, last and location are required, where on MySpace your real name is quite optional. Structured text search and structured results allows you to search for specific information in specific fields, and see the results in content-groups. Friends, Apps, Pages, Networks are all groupings of results by content type. So if you're looking for a Bob Marley Group, you don't have to look through Apps or Profiles to find it.
MySpace was designed to connect people with bands, and enable people to meet online and new friends. The Search function, for better or worse, leverages Google's free-text search to find people based on name, phone number or alias. I'm not sure the choice of Google was thought out from a product perspective, but it was definitely driven by a 3 year Google ad revenue deal.
Using Google's free-text searching finds results based on an entire page of text, so the search picks up main profile info and URL keywords, but also friends' names on the profile, comments, descriptions, etc. This leaves the user with a large, unstructured list of potential matches, which has to be picked through. There's no way to say "the name field is the most important." All results are weighted using Google weighting parameters - inbound links and keyword relevance, the same stuff that's tweaked in search engine optimization. For example, my name might be Tom Kuhr but if if my hero is James Bond and I write about James Bond all over my profile. When a user searches for James Bond - presumably intending to find a friend of that name - my profile will show up much higher in results than James Bonds just because of this keyword repetition, even though that's not the searcher's intent.
There is a Browse function on MySpace, which enables a structured search based on profile parameters, but you can't look for a specific person this way - it only allows browsing on attributes.
Finding a friend becomes immensely easier and faster on Facebook. When I sign up as a new user, I'm able to quickly identify friends and build my lists. On MySpace, I can hunt around in Search, but the experience is challenging and slow and I need to page through results, constantly refine my search, and my network grows much less quickly. I think the use of Google for search on MySpace has contributed to the company's inability to maintain the leadership position as a social network, and is further driving its move to become an "interactive" entertainment company rather than a network.