The power of a large company
Matt Asay is caught by his own argument. He can't see the fact that most of the startups he talks about ("Project Diaspora", "Identica") are things that a lot of techies have never heard of, let alone normal.
If you've never heard of it, how can you sign up to it?
Google+ has the advantage it is by Google, you know that firm that everyone knows about and most people use at least some of it's products. They will integrate and advertise this with their other products and people will sign up out of curiosity.
Also many people are connected to multiple networks: Facebook, linkedin, twitter etc.
Of course this may not be enough to make Google+ the hit that it could be.