The current U.S. life expectancy is 78. The current social security retirement age is 65. The average social security payment is about $1177:
http://ssa-custhelp.ssa.gov/app/answers ... red-worker
That means the total payout, very roughly, is $182,000.
The average U.S. income is currently $43,000 per year. The worker pays 5.3% in social security old age contributions, and his employer pays 5.3%, for a total of 10.6%. Assuming a working life of 40 years, that comes to a total contribution, roughly, of $186,000. So based on current rates and current dollars, you get out roughly what you paid in. It's equivalent to a retirement account that pays interest at the same rate as inflation.
However, that's just a snapshot. Current retirees actually paid in somewhat less, since social security taxes have been going up over the years, so they are getting out more than they paid in. Meanwhile, life expectancy is going up, so current workers are scheduled to get out more than they put in too. Plus, spouses who survive earners also get benefits even if they don't work, so single earner married couples do quite well.
Of course, that all puts social security in the red over the long term, which is why the retirement age may need to be increased, perhaps for people who are currently under 40. There is already an increase scheduled - to 67 - for people who are under 52 or so.
Overall, social security doesn't work terribly as an enforced retirement system, though making the retirement accounts individual and allowing people to track them would be better. The same could probably be said for social security disability insurance.
What really works poorly is medicare, where people get out five times as much as they put in, driven largely by a minority who get huge amounts of medical payments. Medicare is the thing that is truly broken and needs to be fixed.
