The website "Frontporch Republic," is having a symposium on friendship, including this contribution from Susan McWilliams on "Facebook and Friendship."  An excerpt:

What I’d like to stress is that, whatever else you make of Facebook friendship, it underscores the great and significant discrepancy between: 1) the scale of contemporary life, and 2) the scale of friendship.

The scale of contemporary life is so vast that it is hard to fathom. For most of us, completing even the most homebound tasks – using the bathroom, eating a meal – involves us massive networks of pipe and road, transportation and production, people and powers. For most of us, going to work means traveling a fair amount of horizontal distance – and then logging on to connect ourselves to even more far-flung places, to cover an even greater span of space. Ours is an era in which the grand forces are all centrifugal, as William Leach has written, and in which the injunction is to “extend your reach.” We are told to minimize the time we spend doing things – to seek efficiency – in order to extend ourselves further. Services like Facebook are inevitable in this context, since they both allow and encourage the extension of our reach across great, seemingly limitless distances.

By contrast, the scale of friendship is necessarily limited. Friendship is a bounded relationship, one that thrives on intimacy and depth rather than extension and breadth. Friendship thrives, as C.S. Lewis wrote, by withdrawing people from networks of collective “togetherness” into smaller and more partial spheres. Even if, as Lewis says, friendship is the least jealous of loves, it is always to some degree exclusive. Friendship flourishes when given lots of time and little distraction: conditions which you cannot extend to more than a very few people. In the end, the scale of friendship is limited because each of our lives is limited. Our time is limited, and friendship requires time. (It is telling how silly the dominant values expressed in our language sound when they are applied to friendship; no one has ever complemented someone else by calling her an “efficient” friend.)

Put in starker terms, we might say that Facebook friendship is part and parcel of a culture that values a way of living that – while on certain terms quite interesting and rewarding – is inhospitable to the cultivation of real friendship.

Again, here's the whole thing.

McWilliams links to several good essays on the Facebook "friending" phenomenon, but here's one of my favorites:  "Virtual Friendship and the New Narcissism," by Christine Rosen.