If Facebook was trying to launch an integrated search engine or a GMail competitor, then I would absolutely agree with you - those products would be inherently limited by the requirement that they tie into and feed back towards Facebook's core platform, but that isn't the case here.
I see where you are coming from. However, given the potential overhead of running a social network, plus the revenue that might come with it, do you think that Google's priorities might evolve, -that they might morph into a social network, taking their other products with it?