FWIW Facebook tore through my college in a couple weeks way before they had any type of inviting mechanisms. So PM fit was definitely there and virality was all word of mouth.
you guys are totally right (in the growth tactics I definitely tell you lots of the non-organic stuff). That being said the idea here is "how to start a startup" and so I was trying to focus on what got Facebook the first 50million users not the next 1.3Billion and how we think about growth for products at Facebook and how I think about it when advising companies. It's always cool to see the different comment threads (one saying I'm too optimized for big company growth, one saying I'm talking about too early, I am hoping that means I pitched it about right :) )