If there was a penpal network, it probably disappeared due to email, instant messaging, etc. While some people feel that emails remove the "personal" touch, they are much more timely. Many people have a hard time sitting down with a pen and paper to actually write a letter and find emails quicker and easier. Depending on which country you are sending to, it can take quite a while for a letter to arrive.
Since joining TCS, I have become email & PM friends/penpals with people from around the US and the world. Although I have exchanged cards, etc with some of these people, paper and pen have not been necessary to develop the friendships.
I highly recommend becoming a PM/email friend to another TCS member. It is special to be allowed into the life of someone that lives far away from you. While TCS is first and foremost a cat educational and welfare site, the closeknit community and friendships are what keep many people coming back.