I did this a few years ago! I met people from all over the world via Tumblr and we started as pen-pals. I had one in China, Germany, Spain and through several cities across the US.
I will admit, I did lose track of them due to my laziness. I still have all of the letters. I just can't bear to let them go. I'd love to pick it back up once I get settled in at my new job.