Below the 49th virtually every time you turn around you give some third party your SSN. Medicare/Medicaid, most other medical insurance, etc use the SSN as their ID number. I wouldn't be surprised if your local public library requested it when you applied for a card. And of course any bank or credit reporting agency throws it about without regard for privacy.
(Just so you know, north of the 49th the equivalent to the SSN is usually guarded and is certainly not used for the purposes indicated above. In fact, it is illegal for any agency to demand that number unless it is required for tax reporting purposes — ie, bank accounts, paid employment, and the like.)
