Suppose that you're comparing company A, much smaller but has very good penetration in your area, with company B, new to your area but much larger (their home base is, say, Japan). Which do you prefer?
Odds are that you prefer A, because most of the people that you care about are going to be on that network. Despite company B having more people, you're not going to experience more value there.
If you think carefully about that hypothetical, the heart of our criticism of Metcalfe's law is right there.
Incidentally I think that the following paragraph is key:
Metcalfe's Law is intuitively appealing, since our personal estimate of the size of a network is based on uptake of that network among friends and family. Our derived value also varies directly with the metric. We therefore see a linear relationship between the perceived size and value of that network.
Of course this intuition leads us astray because we do not percieve the vast majority of the network, and therefore have bad intuition of what its true size is.
Cheers,
Ben