What are friends for?

In recent conversations with friends there appears to be a common thread. The question arises as to deciding when a friendship changes to being more of an acquaintance or is terminated completely. There is an awareness that the nature of the relationship has shifted or never truly was a friendship. The contacts become further apart and have less substance to them. The interactions are lopsided with one person serving as listener and support while the other talks or complains incessantly about what is happening in their lives. Rarely is the person asked how he or she was doing. We all know there are times when one person in the relationship needs our undivided attention and support. At that point the relationship might be 95:5 in its give and take. But for the most part friendships tend to do best when they average out to 50:50. Each person knows that the other can be counted on to share what is happening in their lives and conversation flows between them in support of that need. It isn't easy to let go of a relationship that no longer matches our understanding of what a friendship is and there is often pain involved in releasing it. When we do it allows for new people to enter our lives. What people have you had to let of that no longer fit the category of "friend" for you and what was that like?