Talking about your own volunteering
If you really want to advocate that atheists do, indeed, volunteer, then one of the things you can do is "witness" for your "belief" by blogging about your own volunteering. You can do this on your online profiles at MySpace, FaceBook, whatever, or on your own blogspace, wherever that is, or on your own web site.
I have three online efforts of my own about my volunteering:
One that lists every where I've volunteered: http://www.coyotecommunications.com/volunteer/asavol.html
One that gives an overview of how people can give time, in-kind donations and money to causes, with some of my own activities as an example: http://www.coyotecommunications.com/cravens/how_I_donate.html
And my MySpace blog is focused on youth volunteering, http://blog.myspace.com/jaynecravens
Just something to think about. It's great to post here, but I think posting via other channels is a great way to "witness" for atheist / secular volunteering.