I've always tried to keep in touch with people, treating it almost like a responsibility. Not to make it sound like a chore; I enjoy reconnecting with friends. But I'm beginning to have mixed feelings about keeping in touch. How can you really know a person if you chat with them for an hour once a year? So many significant events happen in the intervening period. In one hour, you can only hope to exchange some trivial facts ("I got a new job", etc) and you don't have time or energy to explain the important things that happened in your life. So what's the point of keeping in touch? You're fighting like a cliff against the erosion of the sea. Keeping in touch is downright unsatisfying.
One way to think about it is the lottery analogy. One day, you might move to a new town and reconnect with a friend living there. Maybe in the future, you'll end up in a research collaboration with your professor from undergrad. You should keep your options open. A more realistic situation is one where you have trouble making local friends. This could be for a variety of reasons: you're in a tough emotional place, the people around you just aren't that great, there are cultural barriers. Then a nonlocal network of friends is essential to keeping your sanity.
There is also the question of how to keep in touch. I'll describe my current approach. I try to visit my friends who live closeby at least once a year. I send Christmas cards to people and write a personal note in each card (as opposed to the one line "Seasons Greetings". I write a brief update to friends and include some links to my favorite blog posts. I regularly check my Facebook account. I think that's the best I can do. If anyone has better ideas, let me know.