A shockingly high number
Here is one shockingly high number reported by the Department of Labor: in the last year, 61.8 million Americans worked for free, contributing a median of 52 hours, or something like $900-$1000 per person in dollar terms. Furthermore, this number was essentially unchanged from the prior year, suggesting that the first natural suspect - financial crisis - was most likely uncorrelated with the rate of volunteering. Other interesting facts: most volunteers were employed full or part-time, and the rate of participation increases rapidly with one's education level, reaching 42.2% for those with BS degree of higher. It is ironic that this presumably ultra-capitalist society appears to have a stronger commitment to voluntary work than any of those countries that claimed to be building Communism.