Milton Friedman, not too surprising. Paul Samuelson, surprising to me, but he sounds a bit jocular. And Paul Krugman, very surprising to me, who refers to it as having a "Ponzi game aspect."
The best laid Ponzi schemes
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.
end up as broken dreams.