What Social Media Networks do You Belong To?

With names like Twitter, Facebook, Digg, Whrrl and so many more, it's hard to keep track of the "good" social networks vs. the "bad" ones.  Which ones do you belong to and do you find you use them (your choices) for different reasons?  Why?  Example:


I use Twitter to say a quick hello
I use Facebook to help promote my business
I use LinkedIn to establish professional connections
I use Whrrl as a personal journey tracker