I comment sporadically much like how I reply to topics on here.
I try to only comment when I think I have something constructive or interesting to say, but lately I've been thinking of just throwing in my two cents on more comics in an effort to encourage artists and be more into the community. If you flip through series there is definitely a contingent of people who post on a bunch of comics consistently and that might not be a bad way to follow.
I do think some comics are more conducive to comments and discussion than others and it has little to do with whether or not the comics are engaging, just that some have more room for talk. There are plenty of great comics on here that don't have a huge amount of people talking about them.
I'm actually the same way as @joannekwan in regards to posting more in lesser known series. After reading these posts I wonder if just dropping a word or two would make an impact on the community. I mean I know I love reading the few comments I get.