A tongue-in-cheek article, "The 19 kinds of friends" in the October 2001 Utne Reader has the following description:
"The E-mail friend: If the medium is the message, as Marhsall McLuhan claimed, then the message of most e-mail friendships is goofing off at work."
(Incidentally, this issue has articles that people in this forum might like. Article titles such as "Land of the Free... Parking" and "Why Johnny and Jana Can't Walk to School.")