As a guy, I know most of us masturbate, and do it fairly regularly. I've met some guys who say they don't, and while I have no reason to think they're lying, I'm not convinced. I don't think the number's 100%, but I'd bet it's close.
Now, women - I dunno. I know a lot of sex-positive women who freely and openly discuss their masturbation habits, and some of them think that a similar (95%) amount of all women masturbate. In my small-yet-detailed sample set of friends, however, quite a few do not masturbate, and quite a few others do it very, very rarely. Some, of course, do it all the time, and that's fine. That's better than fine, actually.
Based on my life experiences, I'd put it like this:
% men who masturbate at all: 98%
% men who masturbate regularly (once a week): 85%
% men who masturabte daily, when possible: 70-80%
% women who masturbate at all: 75%
% women who masturbate regularly (once a week): 60%
% women who masturabte daily, when possible: 35-40%
So, my question: Ladies, do most other ladies masturbate? Do you? Regularly? Do your men? Do my numbers jive with what you've seen in your life? I haven't looked at any studies/polls/surveys, but I'm curious what the word on the street is.