My close friends and I have been discussing this a great deal recently - about what form we can reasonably expect our friendships to take as we enter into the busiest, most productive years of our lives.
Personal belief: at any one point in time I carry maybe, 1 or two close personal friends. They change, its no big deal. If you are growing, learning, and changing so will your interests, then the people in your life should change. There are a few people(and by few i literally mean maybe 2) who I stay in contact with over a long period, and we can pick up where we left off each time. I would take that as an exception, not the rule.