For me, a true friend is one that stays with me, no matter how far and how often I travel. Even if I don't talk to someone for years, that should not deter either of us from picking up the phone or sending a letter and saying "hello."
The only reason I would ever end a friendship, would be for two-facedness. Don't say we're bff's but then talk bad about me behind my back. But I've let even that one go for a simple apology and a box of chocolate.