You do when you're keeping in touch with friends you've made throughout your life and not just your current ones. That's the thing Facebook makes easy.
Also email and writing letters don't really work. If I sent a letter to a friend I seriously doubt they would bother responding, and most people I know personally don't use email. They use Facebook. That's the big problem - Facebook has replaced email for a lot of people making it one of the only ways (besides visiting/calling) to contact them.