There have been various discussion on Linked-in recently about whether Communites of Practice, and other networks, help to retain staff.
Below is some evidence from Deutsche Bank that this is the case, quoted from here
In 2000, the acquisition of Bankers’ Trust prompted an exodus of key investment bankers — taking accounts with them. However, top managers were less concerned by the loss of accounts than by the loss of knowledge, which they feared could potentially have even more severe consequences. After all, the managers who quit the organization had an in-depth understanding of key procedures and they knew best how to manage their customer relationships. In 2000, Deutsche Bank decided that merely tracking turnover was not sufficient, so it made explicit efforts to develop an indicator to measure the commitment of key individuals to the company. The reasons individuals left the bank proved the point that best-practice networks are one of the most important tools for tying highly qualified managers to the organization.