David DiSalvo's excellent Neuronarrative science blog brings us news of a sociological study on friendship. Though intended to discover where and how people meet their friends, it also accidentally discovered that the average person replaces roughly half their friends in seven years.

The results: personal network sizes remained stable, but many members of the network were new. Only 30 percent of the original ‘helper’ friends and discussion partners had the same position in a subject’s network seven years later, and only 48 percent were still part of the social network.

That means there's no need to feel guilty or bad about drifting away from your friends or losing them - that seems to be part of the normal human experience, at least in modern times. Now, if you only have half as many friends as you did seven years ago that probably is a problem and you should seek out some opportunities to make new ones (in other words, get a life), but most people naturally replace their lost friends with new ones. Social network size is remarkably stable, and as others have discovered even modern social networking technologies don't seem to have any significant effect on it.

So friends aren't really forever as individuals, but some form of a group of friends you can rely on should always be there. You never know what you'll need them for. Remember the old saying: "Friends help you move; real friends help you move the body".