each person could effectively cause $20,000 to be generated out of nowhere

As a rationalist, when you see a strange number like this, you have to ask yourself: Did I really just discover a way to make lots of money very efficiently? Or could it be that there was a mistake in my arithmetic somewhere?

That one billion dollars is not being generated out of nowhere. It is being generated as payment for ad clicks.
Let's check your assumptions: How much money will the average user generate from banner ad clicks in five years? How many users does Craigslist have? What fraction of those users would have to request banner ads, for Craigslist to add them?

My completely uneducated guess is 100$, ten million, and 50%. This matches your "generate one billion dollars" number but suggests that critical mass would be five million rather than fifty thousand. Note, also, that Facebook users are not necessarily Craigslist users.

I would be interested to hear what numbers you are using. Mine could easily be wrong.