I've been laughably lucky when it comes to friendship. I'm still close to a friend I've known since preschool, three from my elementary years, eight from high school, four from college, and several more from my early years in the workforce.
Beyond this core group of people whom I'm sure I'll know until old age, for most of my 20s, I also worked hard to establish and maintain new friendships, whether with coworkers, colleagues from other companies, friends of friends with whom I felt I clicked, and so on. And I held onto friendships from days past that had never been that deep, but instead existed only because of shared history.
Now well into my 30s, I've accepted the fact that you just can't be all things to all people. Now, any friendship I feel fading, I gently allow to go.
If you feel like you're putting in all the effort or if you notice you and your former friend simply don't contact each other anymore, it's usually best to accept the reality of the situation. I'll gladly strike things back up with old acquaintances, grabbing a beer or coffee and talking of times past, but I will no longer work to maintain a relationship just because it was once so.