It's as close as you can come to a promise without actually being one. My SS statement, which I just got the other day, sure makes it sounds like a promise.
But that's not the point of what I was saying. The point is where that additional money comes from. In an annuity or insurance policy, the extra wealth that I would get comes from wealth generated by the insurance company (if the company is run properly, anyway).
And in theory, so does the money for the SS -- in the so-called "social security trust fund." Paid-in contributions that exceed current liabilities are used, and have historically been used, to buy government securities and the income from those securities has been used to generate additional money to fund future liabilities. I believe the rate of return is around 5-6%, which is really rather a sweetheart deal.
There were about $2 trillion dollars in this fund as of late 2006; I don't have more recent figures to hand, nor are they really relevant. More relevant is the fact that the fund is expected to
grow for the next decade or two, precisely because the demographics are still in our favor. Only after about 2020-2025 are we expected to have to tap the trust fund.
Now, you might suggest that government bonds are a stupid spot for the government to park excess money. That suggestion has, in fact, been made. But that's not the same as saying that the money doesn't exist -- that money is just as "real" as the T-bills that banks have been flocking to buy in the past several months, and if the banks are to be believed, then it's probably better and safer than buying anything on the commercial market.
At this level of detail, social security is probably in much better shape than your own pension fund, especially with the level of equity risk that we've seen in the market right now, with more than two trillion dollars of reserve capital parked in essentially risk-free investments.