The Online Family answers:

One of the most popular social networks for professionals is LinkedIn. It allows you to post a resume-like profile and to connect to people you used to work with at various employers. Every time you log on, you'll see the names of people who've also listed those employers and be able to add them as connections if you know them. Plus, via the site, you can ask your former co-workers to link you with their connections as well. To use LinkedIn's more advanced features -- such as directly emailing friends of friends through the site -- you'll need to pay a subscription fee starting at $19.95 per month.

Another option for staying in touch is Facebook, which isn't just for college kids anymore. To pick the best online network for you, check with a few of your closest or most important colleagues to see which sites they already use.