LinkedIn (http://www.linkedin.com) is a popular BusinessNetworking site (their main, and much smaller, competitor is Ryze.com) and gaining lots of acceptance recently.
LinkedIn manages the "network of contacts" that some professionals use by instinct and others need help with. -- PhlIp
It is a lot less fun than FaceBook, but then again, its not meant to be. Hmmm... when did it happen that work is not any fun (see FortyHoursVsFun)?