I am amazed at even MySpace how many good URL user names are still available. An example is http://www.myspace.com/kinlane/ which is the URL representation of your MySpace profile.
It is much like securing the domain name for your company. You should go to all major and even some minor social networks and sites and secure your username.
Even if you don't actively use the account, make sure and secure it so that you have the name and login for the future. You never know when a network or application will go viral and having your company name or your industry / product name can become very valuable.
Think if you had http://www.myspace.com/traveling/ or something similar. It would be very valuable.