I argued in a previous post that customer defection is not necessarily the catastrophe it might seem to be. Customers can “defect” whilst still maintaining a positive attitude about a company, which means they are both good prospects for the future and a potential source of positive comment about you.
Much the same should be true of staff. If you treat staff well then, when they part company with you,they should retain positive attitudes about you. These will make them more likely to use you as a customer, if applicable, and will also affect the way they talk about you.
One of the best examples I've seen of this is the ex-employee of US Airways who posts on Twitter as http://twitter.com/usairwaysgirl. She looks out for tweets that mention the airline, positively or negatively, and replies. Often she tries to help people find a solution to a problem that they've experienced.
Frankly this is something that US Airways, and other consumer brands, should be doing much more actively anyway. Until then, usairwaysgirl is doing her best to preserve her ex-employers reputation. That's testament to unusual levels of loyalty.