Research released from the National Institute on Retirement Security provides some stark data on the extent to which Americans are relying on Social Security for their economic well-being in retirement. Using data from the Federal Reserve's Survey of Consumer Finance, they estimate that 38 million households do not have any assets in retirement accounts. The full study is available here.
Now some of these differences may be attributed to definitions -- note, for example, that the report focuses on assets in retirement accounts. If you aren't using a tax-deferred IRA or 401(k) or 403(b) plan your assets -- which could be considerable -- wouldn't count as retirement account assets. Nonetheless, this result does paint a rather dismal picture.
As the chart above -- taken from the report -- illustrates, even those closest to retirement often have little set aside in retirement savings.