After Tim O’reilly talked on network effects..here are some good articles on this topic

good book from Shapiro and Varian

Information Rules. A Strategic Guide to the Network Economy

http://www.amazon.de/Information-Rules-Strategic-Network-Economy/dp/087584863X

Scientific article from Shapiro and Varian

http://www.inforules.com/models/m-net.pdf.pdf

Nice overview

http://oz.stern.nyu.edu/io/network.html

Book from shy Oz: The Economics of Network Industries

http://www.ozshy.com/

16 pages pdf on that

http://assets.cambridge.org/97805218/00952/sample/9780521800952ws.pdf

Carl Shapiro :
COMPETITION POLICY IN THE INFORMATION ECONOMY

http://faculty.haas.berkeley.edu/shapiro/comppolicy.pdf