It's not just the case with long distance friendships
Interesting subject. I've never been good at keeping friends. Nor making them in the first place. I had a good group of friends in my previous college. After graduation, it all faded.
Strangely, Carmen, my best friend in the whole world, lives far away from me. We grew up in the same city, but I moved away when I was 10. Yet we always kept in touch. I pulled her through shitty situations, she helped me with mine. This summer we're going on vacation together.
I think that the main reason friendships fade is because most friendships are superficial. Take a look in your group of friends. How many would go through fire for you? Would you sacrifice for them? For 1, 2 or 3, maybe.
Also, the world has become a lot smaller. You have lots of people at your disposal today. Here I am, talking online on this forum, occupying myself with the problems of people who have nicknames like "crispychicken." What I mean by this is that we don't need to cling to people close by anymore because we can meet people everywhere. I think it makes people sloppy in a way. Networking is popular. Facebook, Hyves, Myspace, all those sites are there to show off your friends list and connect with other people.
However, the more contacts you have, the more superficial the contact becomes. Friends become categorized. Nowadays many people have friends they shop with, other friends to have lunch and talk with, that one you maintain a site with... We're like spoiled children. We got have so many shiny toys it isn't special anymore. A commando who is sent to a war zone with nothing to depend on but his squad buddies forms a much deeper relationship with them than most of us do with our friends.
That's my take on it. Our means of communication are improved. Because we know we can call that person via Skype, use MSN or e-mail them, we'll settle for doing that tomorrow. Or the day after. Or the week after. Or a month.