If you haven't heard of Thirty Day Challenge, you may want to check it out. In 30 Days, it teaches an individual how to set up a blog-based business and establish a comprehensive web presence using various social media tools...all free.
If you ignore the part about setting up a web-based business, the "pre-game" alone is worth browsing as it introduces you to Flock (per Steve's suggestion above), FriendFeed, Squidoo and much more...once you've had this orientation to the breadth of social media options on the web, you'll be more than prepared to narrow it down to 3-4 that maximize your experience/presence.
So...after completing the Challenge, I narrowed it down to (in no particular order!):
1. Facebook (keeping in touch with friends and family)
2. LinkedIn (making professional contacts and learning from groups/forums)
3. Twitter (staying connected, getting news/ideas - as a teleworker, this is my "water cooler" community)
4. GovLoop (sharing knowledge and ideas, networking, cross-posting from blog)
FriendFeed updates my social bookmarking, Twitter and Facebook activity, though I don't pay attention to it that closely. Flock is also very handy to keep everything straight!