I often buy postcards while on vacation (including when I am on a cruise ship) and mail them to family, friends, and strangers. If I am on a cruise, I wait until I get home to mail them.
With the strangers that I mail them too, I know a few teachers from online that uses postcards in the classroom who uses postcards to help teach history, geography, est. I am also into a hobby called Postcrossing, an online project for people to exchange postcards with other members globally.