Eventually, though, and this might take a long time, but it is finite,
everyone will have tried Facebook ads and know that they are useless.
Eventually, after 10 million businesses have invested $1000 each, and
Facebook has earned $10 billion in revenue in total, then they will
have run out of new customers and their revenue will dry up.  A
useless product is never sustainable.  I wish I could short Facebook.

Although I think calling Facebook a Ponzi scheme is a bit of an exaggeration (I don't think they're out to swindle anybody) I do agree that Facebook advertising just doesn't seem sustainable the way Google ads are.

Everyone likes to think they can make millions from ads like Google does, but the reality is Google is in a very specific situation: search engines are one of the few kinds of sites that people go to planning to find information and then *leave*. Any site that people want to stay on is going to have a hard time making real money from PPC ads.