Dear Sonja,
welcome to Socnet, I am a phd student myself at the University of
Toronto. The question that you are asking is of most relevance, but
difficult to answer theoretically, operationally and empirically.
I have done some reading about both formal and informal networks
because I am interested in informal networks.

My sense from the literature is that formal communication is either

a) something abstract depicted in organizational charts that does not
capture what really goes on in organizations.
Formal networks are related to formal organizational structure and
thus some definitions include values, roles, and beliefs.

b) it influences informal communication, but it is unclear how: maybe
through the values, roles, beliefs and expectations that formal
structures have. (see any textbook).
Mintzberg, H. (1994). The structuring of organizations. Englewood Cliffs, NJ, Prentice Hall.
Scott, R. (1998). Organizations: Rational, Natural and Open Systems. Englewood Cliffs, NJ, Prentice Hall.

c) in information science we often define formal structure as the
documents, memos, and meetings, etc. that occurr in an organization
(see for example Choo, 1999).
Choo, C. W. (1996). Towards an information model of organizations. Managing Information for the Competitive
Edge. E. Auster and C. W. Choo. New York, NY, Neal-Schuman Publishers, Inc.: 7-40.
        , C. W. (1998). Information Management for the Intelligent Organization: The Art of Scanning the
Environment. Medford, NJ, Published for the American Society for Information Science by Information Today.
        , C. W. (1998). The Knowing Organization: How Organizations Use Information to Construct Meaning,
Create Knowledge, and Make Decisions. New York, NY, Oxford University Press.

An excellent discussion on how the two may influence one another
can be found in Monge & Contractor.
Monge, P. R. and N. S. Contractor (1988). Communication networks: measuring techniques. A Handbook for the
study of human communication. C. H. Tardy. Norwood, NJ, Ablex: 107-38.
Monge, P. R. and N. S. Contractor (1997). Emergence of communication networks. Handbook of Organizational
Communication. F. M. Jablin and L. L. Putnam. Thousand Oaks, CA, Sage Publications.

Also helpful may be discussions on network organizations (which is the area that I am interested in).
Baker, W. (1992). The Network Organization in Theory and Practice. Networks and Organizations: Structure,
Form and Action. N. Nohria and R. Eccles. Boston, Harvard Business School Press: 397-429.
Hage    , J., Ed. (1988). Futures of organizations. Lexington, MA, Lexington Books.
Jarvenpaa, S. L. and B. Ives (1994). "The global network organization of the future: Information management
opportunities and challenges." Journal of Management Information Systems 10(4): 25-57.
Miles, R. E. and C. C. Snow (1986). "Organizations: New concepts for new forms." California Management
Review 28(Summer): 62-73.
Miles, R. E. and C. C. Snow (1992). "Causes of failure in network organizations." California Management Review
28: 53-72.
Ward, R., G. Wamsley, et al. (2000). "Network organizational development in the public sector: A case study of
the federal emergency management administration (FEMA)." Journal of the American Society of Information
Science 51(11): 1018-1032.

Also, of interest may be the excellent dissertation by Nancy Nazer, who worked under the supervision of Barry
Wellman and studied a "virtual organization". Her thesis includes a chapter comparing virtual, networked, and
bureaucratic organizations. She also published an article.

Koku, E., N. Nazer, & Wellmman, B. (2001). "Netting scholars: Online and offline." American Behavioral Scientist
43(Special issue: Mapping Globalization): 1750-1772.

Nazer   , N. (2000). The Emergence of a Virtual Research Organization: How an Invisible College Becomes
Visible. Department of Sociology. Toronto, University of Toronto.

I have a section in my thesis discussing formal and informal networks that  I could forward you.
Good luck!
Anabel

Many Regards,
         Anabel Quan Haase
 ----------------------------------
 Ph.D. Student
 Faculty of Information Studies
 McLuhan Program in Culture and Technology
 University of Toronto
 140 St. George Street
 Toronto, Ontario
 M5S 3G6
 Tel: 416-978-7097
 Fax: 416-971-1399