[This is the only part of the metaphor worth reading] There is one warning, however — Facebook's hegemony will seem impregnable right up to the moment it collapses. Because the attraciveness of these sites depends on the number of other users, there's always the possibility that an inflection point is reached in which everyone migrates from Facebook to Orkut or something else. And when that does happen, the fall of Facebook will be fast and hard.