SCTNet (http://SCTNet.edu.tw) has been the largest virtual professional community of teachers in Taiwan and serves as the knowledge management platform that supports teachers to codify, store and share their knowledge. With the endorsement of teachers and support of government, SCTNet was expected to be successful. The fact that over 90,000 teachers have registered to become SCTNet members so far was also a testimony to this expectation. Yet, the actual usage over four years has been disappointing. To investigate this phenomenon, we have conducted a two-phase study over a two-year period: first a survey study based on the Theory of Planned Behavior, and second, a qualitative study of over 40 SCTNet members. This presentation reports the results and implications of this two-phase study.