That’s the claim made by Paul Krugman, among others.
But it’s not true when you count the benefits they generate, argue Kip Hagopian and Lee Ohanian in the Wall Street Journal. While taxes are a flat rate capped at earnings of around $110,000, benefits are paid on a progressive basis.
“Given the design of Social Security, the only way the program could be regressive is if the mortality rate differences between the group of higher-income and lower-income workers were so large that higher-income people received greater lifetime benefits for each dollar contributed. But this is not the case, according to a 2009 study by the Social Security Administration's Office of Retirement and Disability Policy. Rather, the study concludes that ‘Social Security is modestly progressive on a lifetime basis; currently, the program lies approximately halfway between paying a benefit directly proportional to lifetime taxable earnings and paying a flat dollar benefit to each retiree.’"
For anyone interested, the SSA study was written by me, Mark Sarney and Chris Tamborini during the time I was at the agency. Both Mark and Chris continue their good work at Social Security.