What is a good ratio of active users to total signups for a free product?

First you get someone on to your website, then with a good landing page you persuade them to put in their email and password to signup, then they confirm their email address to rule out spammers etc. This already leaves you at a fraction of the audience that you started with.

But are there any rules of thumb for what is a good ratio of people actually using the product compared to how many signup?
I'm hovering at about 30-40% at the moment for holfolio.com (still small absolute numbers), and it has been going down recently which is a slight cause for concern.

I'd love to know whether a standard benchmark should be 10% or 80% to have an idea whether I should focus on this more at the moment.