Facebook, MySpace, Twitter, LinkedIn,an unused blog and a podcast website. Apart from the blog, the rest were set up as part of the podcast identity, though I rarely use Facebook and Twitter is more personal than podcast-related. Bands and musicians all have their own favourite ways of contacting you via the web, so having a presence in the main social networking sites is almost a requirement to allow them to get in touch via their medium of choice.
Couldn't see the point of Twitter when I signed up in April 2007 when my fellow podcasters were all getting in to it. Now I love it. Keeps me amused, entertained & informed!