SNA expert Robert Cross writes about Knowledge Loss in Organizations at Visible Path:
In short, departure of key people--not just those high in the hierarchy but those central to the inner workings of a network--can significantly impact the relationship structure and consequent functioning of an organization. Social network analysis (SNA) can inform knowledge retention efforts by allowing managers to visualize the myriad and often invisible relationships that exist behind the formal organizational chart but are critical to getting work done.
This paragraph is written after several that describe the general problem organizations are facing with knowledge retention / knowledge loss as people are set to retire in greater numbers in the coming years. (Even those who are far from retirement stay in their jobs for shorter and shorter periods, but I suspect they are building fewer central roles in the social fabric of organizations.)
I like the emphasis on using SNA as a diagnostic tool, particularly as he talks about the differing impacts of Central Connectors, Brokers, and Peripheral Players. Along with their different characteristics, I suspect that different interventions are required to buffer their loss.
This topic is also interesting because a couple of the masters students I'm advising at Northwestern are looking into the question of how the social fabric changes as people leave or enter the larger network. Rather than using SNA directly, they are looking at the question of knowledge flows and "socially constructed knowledge." An image I have in my head is that of an amoeba: when someone leaves they remove not only themselves but also their linkages throughout the entity, changing it. Similarly, when a new person arrives, they will spend time integrating themselves into the entity, also creating change. (I do a much better job with this description in person with a lot of hand waving.)