I used to have a Myspace but when Facebook started to become popular, I had move because all of my friends had a Facebook and they were barely active on Myspace anymore, so they told me to create an account. I'm quiet active nowadays around Facebook, it's nice to be part of a social network.
As for Twitter, I have an account, I don't 'twit', I just follow bands, websites, etc. to see what's up.