The beauty of networks is the fact of their existence. Many people who leave organizations continue weak and strong ties in networks of organizational alumni. Many people continue to grow in all kinds of wonderful ways.
Organizations have two choices. They can act as if they don't exist, or they can nurture the growth of these networks and tap into the rich resources for its growth. This can be done by any teams. It doesn't have to be a centralized or formal function. It's an incredible gift to engage rather than deny.