From Ezra Klein at the Washington Post:
Earlier, I criticized Alan Simpson for not knowing — and, more to the point, being actively hostile to — accurate demographic data about Social Security, but perhaps it’d be more useful to run through some of the numbers I find it helpful to keep in mind while writing about the issue:
1) Over the next 75 years, Social Security’s shortfall is equal to about 0.7 percent of GDP. Source (PDF).
2) For the average 65-year-old retiring in 2010, Social Security replaced about 40 percent of working-age earnings. That “replacement rate” is scheduled to fall to 31 percent in the coming decades. Source.
3) Social Security’s replacement rate puts it 26th among 30 Organization for Economic Cooperation and Development nations for workers with average earnings. Source.
4) Without Social Security, 45 percent of seniors would be under the poverty line. With Social Security, 10 percent of seniors are under the poverty line. Source.
Social Security Works - Washington 