Can I continue to contribute to U.S. Social Security after I leave the U.S. due to the expiration of my H1b visa to collect some small pension at retirement?
Thanks,

No, you would have to be a resident alien to be liable for social security taxes on your earned income. Voluntary contributions are not permitted. However, some countries have bilateral agreements and recognize the time you paid into US social security. http://www.ssa.gov/international/totalization_agreements.html