Geek Girl Pen Pals is the solution to your woes. Users can sign up to be paired with a pen pal from anywhere in the world and exchange postcards, letters, or packages of all kinds. Despite the name it’s not limited to girls, and you can specify if you only feel comfortable being paired with pen pals of the same gender.
Every month a new ’round’ begins on the 15th. Users will only be paired with one pen pal per round, but they are free to apply during the new round each month for another pen pal. When applying, one submits a list of 5 interests, and the site admins will try to pair the applicant with someone who shares as many of the same interests as possible.
There are four age groups: 13-17, 18-22, 23-29, 30+. Users will always be paired with pals in their age group. (I won’t comment on how old it makes me feel that everyone over the age of 30 is lumped into a single bracket; I’ve accepted that once you’re past 29, young people categorize you as practically ready for a senior home.)
There are themed months, with the upcoming October-December themes being Dark Fairly Tales, We Love The 90s, and Time Travel.