Efficiency is the enemy for things like this, because it only shows a minimum amount of involvement with the issue. You show you care at all by participating in petitions, but you prove you care more by participating in more time-intensive activities.

This is something that you'll see people close to politicians say quite a lot, that the method of communication counts, and a multiplier is used to count your input based on that method. It's why you're supposed to write to your $political_representative on paper rather that firing off an email, and it's why you're supposed to hand write the letter (legibly, please!) rather than typing it. A person who takes an hour or two drafting a compelling response to legislation, writing a final copy by hand, taking it to the post office, affixing a stamp, and mailing it out has shown more concern for the issue than does a guy who goes on HN, sees this link, and fills out a form to effectively just say "yeah, me too, what he said!"