Woo. Let's chill out a bit.
1. I actually do like Facebook, and am very impressed by what they are pulling off.
2. I learned about Ponzi schemes a while back in macro theory class, before they were mainstream and irremediably tainted by the vileness of Bernie Madoff. The story of Ponzi himself -- as I remember our macro professor telling it five years ago -- is pretty 'funny', or at least provided welcome distraction between two utility maximization derivations in econ class; he's supposed to have been a 16th century Venitian who ended up in the Venice lagoon once the bankers he was pulling his cash from realized what happened. I didn't think of it as a crime. All this to say that I never implied nor intended to imply that Facebook was committing any sort of fraud, immoral behavior or else.
3. What I mean by this derivative thing is quite simple -- imagine the movie Speed with a bomb that explodes, not when the speed goes under X mph, but when the bus's acceleration goes below X mph/h, forcing the bus to keep on accelerating, driving faster and faster and faster. Eventually you'll hit the physical speed limit of the bus, acceleration will converge to zero, and the bus will explode. That's a very silly metaphor for what I was trying to explain - being that, if indeed FB had to rely on an ever growing in-flow of first time ad buyers to keep the lights up, they are would be in as desperate a situation as was Mr Ponzi in 16th century Venice.
Just to cool everyone's mind -- I really don't think this will happen. The team is least extremely smart to have gotten where they are, and I trust Facebook to eventually find as ridiculously profitable a business model as AdSense, with high probability. As a user I do hope it will respect my privacy, and as an entrepreneur I do hope it will allow some cool new distribution models.