Facebook - A great platform for keeping up with (stalking) friends. The ability to share and communicate with friends and colleagues is seamless. Overall, Facebook is a great place to start your social networking endeavors.

LinkedIn - A must-have in every business person's arsenal of tools for marketing oneself and neworking with ease.

Twitter - Share, Learn, Meet.  Within the first half of 2009, Twitter has gone from nearly 5 million users to over 23 million.