If Facebook wants to maintain its position as the silo for the vast majority of our social data (at least in the Western world), it must focus more on becoming the plumbing of the social web. In other words, more of a platform and less of a destination website.
It can’t do both brilliantly. If it tries, I predict it will eventually go the way of AOL and Compuserve before it. They held on to their walled garden model too tightly and didn’t evolve radically enough. They died as epicentres of the web as a consequence.