Metcalfe's law also doesn't take into account for market saturation. (what happens when everyone is part of the network). It assumes that growth can go forever, but there is only a limited number of people in the world.
Metcalfe's law can be a good approximation provided that the network costs are low. For instance Facebook where they have very few employees compared to a typical company of their size.
The thing that will throw both laws out the window is that both assume price for goods/services remain constant. We don't have that here. A year ago the price topped out over $8 for STEEM. That STEEM was being used to cover operating costs of Steemit.com and Steemit Inc (i.e., being traded for goods and services). Now it is less than 40 cents. Our operating cost fluctuates with price of our currency. The problem with both laws is that you can't add a variable that accounts for price of STEEM as there is no way to predict the price of STEEM.