You can build 'a facebook' in Drupal which is more or less functionally equivalent and people have been doing that for years. But you still have the real work - building the network of users, which is where the value lies and not the software features (which are mundane in the case of fb).

What's far more compelling but challenging is open source federated social networking, which is a facebook on rocket fuel and overcomes the network effect as each provider implementing adds to a shared network. Even if it takes decades, this is going to be the inevitable model and fb will end up either joining in as a provider or do a myspace.