Below quote is from SS Website
>The Social Security Administration has a letter forwarding service that can be used to attempt to contact a missing person. We can attempt to forward a letter to a missing person under circumstances involving a matter of great importance, such as a death or serious illness in the missing person's immediate family, or a sizeable amount of money that is due the missing person. Also, the circumstances must concern a matter about which the missing person is unaware and would undoubtedly want to be informed. Because this service is not related in any way to a Social Security program, its use must be limited so that it does not interfere with our regular program activities. <

Try going to below address or home page of US.Social Security Administration and look for the FAQ (frequently asked questions) or search their website using words: letter forwarding
http://ssa-custhelp.ssa.gov/