Even if Facebook collapses, it doesn't mean it will take the rest of the web down with it. It just means that perhaps a place where you are shooting the breeze with your beer-drinking buddies isn't the best place to sell some, perhaps even most, goods and services. If that's the case, and the price for putting an ad on FB decreases, it doesn't following that the price for running an ad on another website will also be lower.
The author making the fundamental mistake of assuming that all web sites are the same, and that's obviously false. Just because Facebook is (in his view) a ad-supported website doesn't mean that it is interchangeable with any other ad-supported website.