There is a chapter in Wellman and Haythornthwaite's _The Internet in Everyday Life_ by Copher in which she had people in an organization keep communication diaries. She found that those who used CMC more at work communicated MORE face-to-face and on the telephone with their co-workers. This included interpersonal communication as well as strictly task oriented.
Thank you Nancy. This is precisely the kind of studies I am looking for. Note that in the study you mention, the fact that CMC use seems to be optional is important; in the case my students are interested in (computerization of telephone customer service in a large organization), employees were forced to substitute telephone and F2F interactions with CMC interactions (probably through a dedicated event management application). I wonder if researchers in the field of cyberpsychology would have their own findings about this question? I have found something related to the topic in J.B. WALTHER (1992), "Interpersonnal effects in computer-mediated interaction", but the focus is on the psychological features of mediated interaction, not the overall psychological incidence of mediated communication in the organization. Guillaume