Of course, it's a Ponzi scheme. A ponzi scheme works until you run out of new investors, and that's pretty much what's happening now. There are less new contributors than there are recipients. They are going to have to cut back benefits. Obama, Romney and Perry have all admitted that we have to cut SSI benefits, because the current system isn't sustainable.
I agree that we have to reform SS but you are misinformed on a few fronts, as are most people using this rhetoric, including Rick Perry. (Didn't he get a C in his Econ101 class?)
First, a Ponzi Scheme by definition means there is no economic basis for the returns promised, and usually the returns promised are extraordinarily high. A Ponzi Scheme promises a (usually high) rate of return that is paid entirely out of those who are giving their money to participate. That does not apply to SS because SS is funded out of taxes derived from the economy. When the economy grows, so do the contributions into the fund. Unlike a Ponzi Scheme, which has no fundamental economic basis to promise the returns, the US government has the power to tax economic activity. Thus the power to fund SS is derived from the growth in the economy. You can - like Reagan - reform SS by taxing people more if that's what we so choose. You cannot fund a Ponzi Scheme in any other way than sucking people into the Scheme.
Also, unlike a Ponzi Scheme which promises people returns of 50% or 100% returns, or even higher - that's how they get people to participate in the scheme - SS is crediting people's accounts at a mix of US government bond interest rates, or about 4% today. You aren't going to get anyone into a Ponzi Scheme by promising them returns of 3.75%.
Also, you are incorrect to characterize that we are "running out of new investors." The population continues to grow. The economy continues to grow. That there are fewer
new contributors to total recipients is irrelevant because there are
always fewer new contributors to recipients. The ratio of
total contributors to recipients is still 3:1-4:1. The reason why we have had a dip in the surplus is not because "its bankrupt" but because of the recession. Unless you think the US is growing to grow at 1% a year for the next 3 decades - which it won't - then we will revert back to trend when the economy resumes its long-term trajectory. SS is funded out the growth in the general economy. As long as the economy is growing, there is no problem funding SS. You may have to reform it - which we do - but it is not in structural decline.
The reason why we have to cut back benefits is not because its a "Ponzi Scheme." It's because we are living longer than we currently thought when we created and reformed the system. This is a graph of population projections at various points in time in the UK, but it is similar to what happened in the US.