I personally never thought of using social networking to build a database of potential employees. It definitely makes sense.

For more insight in how to implement such a strategy INC online has a good article by Colleen Aylward, here is a sneak preview:

Social networking services can help you build a database of potential employees and your business's brand at the same time.

Social networks have multiplied in recent years as leading tools for finding and connecting with likeminded individuals. And though the "social" in social networks seems to speak more to making new friends, these networks are equally important to businesses, which can leverage them to find new employees. The use of social networking tools for business purposes is simply called business social networking, and it's a process by which you can search and communicate with people who have similar interests and similar goals as you and your business.

Learn more about using Social Networking tools as a recruiting strategy.