I regularly use the following social networking sites:

• Facebook
• LinkedIn

I'm on MySpace too, but seriously, it's only for tracking real-life musicians and people who keep their blogs there. If you are a MySpacer, surely you can find my profile without any further assistance.

My micro-blogging presence:

• Jaiku -- also aggregates my blogs and most other online presences
• identi.ca -- open micro-blogging
  
• Twitter

And on social recommendation sites

• Delicious bookmarks
• Last.fm
• StumbleUpon