I actually think Facebook has a decent grip of this spam problem. Your news feed doesn't show you all the events that happen, only a limited set. They figure out which people you interact with more, which updates get more attention, etc and only show you the ones that they think matters to you.

Their goal is to capture as much data as possible (updates, likes, places, pages) and to find the connections between things. With enough data and number crunching, they can figure out if you're interested in xbox or Mafia Wars updates or now and adjust accordingly.