I'd say around 5-10% have commented recently although it's around 30% long term followers have commented on a hub or more.
I personally tend to comment on a hub when I see some specific reason to comment. I read hubs which are generally of interest to me (sport, fitness, travel and occasional food/ drink hubs) so if the people i'm following go off on a tangent to that there's a likelihood I won't read unless the concept appeals to me in some way.
It actually annoys me when I get a 'nice hub, voted up' comment because it doesn't add anything to any arguments or any insight into the area.
Unfortunately I often criticise hubs as well if I feel duff advice is given out or if they look poor. I know my work is not perfect but it's targetted and there's reasoning behind it all