This is a point that I am always curious/skeptical about because growth will always become marginally harder. So ads, marketing and PR are always going to be necessary at some point.
Using his examples, facebook is creating airplanes to deliver facebook to people they can't reach otherwise - talk about high acquisition cost. AirBnB has a ton of TV [2] and Print ads.
Further, companies like abcmouse [3], dealdash [4] all blanket the airwaves with their ads and seem to be doing well.
So maybe some companies/products lend more to traditional ad spend than others, or maybe the idea that viral is the only way to grow and sustain is wrong.
[1] https://www.facebook.com/notes/mark-zuckerberg/the-technolog...
[2] https://www.ispot.tv/brands/oLp/airbnb