As a musician, I'll have to go with MySpace. It's still the best way to connect to my audience and other musicians. What I like about it the best is that it's topic specific, so it fits my purpose.
As for networking outside of the musical spectrum, I like Twitter. It's quick, precise and easy. It doesn't have the bells and whistles of other social networks, but that only works for it's advantage since it makes it's use more focused and the connections more immediate.