I really like Twitter. It's social networking at the right speed. Faster and far more versatile than Facebook, but slower and much more cosmopolitan than IM.
Twitter is especially cool for people like me who telecommute most or all of the time. I don't have an office community I'm a part of on any regular basis. Twitter provides some of the same social experiences but in a much broader social context.
For non-Twitterers, go watch this 2-minute video on what it's about: Twitter in Plain English
If you use Twitter on a Mac, I love Twitterific