It is not literally a ponzi scheme, as I state very directly in the article. The point is that it is bad in exactly the same way that false returns in ponzi schemes are and through similar mechanisms.

Paul Graham takes the same line of reasoning in declaring that Yahoo is a "de facto ponzi scheme". http://www.paulgraham.com/yahoo.html .

Finally, and I didn't write this in the article (yet), but it's clear that it's acting much more like a literal ponzi scheme with the recent Goldman investment. Accel partners has mostly cashed out to new investors (http://techcrunch.com/2010/11/19/accel-facebook-chunks-of-st...), and I personally know of some major top investors who have cashed out of most of their stock in Facebook. They rode the wave and have sold on to new investors. Goldman is (literally) banking on the public investing later at an even higher valuation at the IPO they will be running themselves.