Ramesh Ponnuru tires of the debate:

Social Security, unlike a Ponzi scheme, is not run for the financial profit of the people who run it; in fact, the government is running it at a loss, and that loss is projected to increase. Participating in Social Security, unlike a Ponzi scheme, is involuntary. And Social Security, unlike a Ponzi scheme, can be reformed so that it becomes a sustainable program. How to do that is probably what we should be talking about, instead of the validity and limits of an analogy.

Michael Lind agrees that the analogy is fatally flawed. As does Matt Steinglass. Alex Tabarrok makes a distinction:

Social Security is not necessarily a Ponzi scheme but it only generated massive returns in the past because of its Ponzi-like aspects. The Ponzi-like aspects are now over and social security is turning into what is essentially a forced savings/welfare program with, as Krugman recognizes, crummy returns for average workers. Social security is thus a Ponzi scheme which has not gone bust but it has gone flat.

Earlier posts in this thread here, here, here and here.