As @Kittosaurus said, use the description area to engage with readers! Asking a question is one way, but you can also comment on the current page (as you have), talk about what you were thinking about when you made the page, what you're happy with, what you struggled with (don't be to negative).
If you have time, you might also create extras to celebrate holidays, comic landmarks like reaching 50 subscribers, and rewards for tippers when you reach that level of growth.
Also, be patient! Growth can happen very slowly, and only a small percentage of our readers are going to be the sort to engage, so while you do your best to get that number up, don't be discouraged if it takes time.
You're in the unique position of posting your comic after you've finished it, so other than promotions, extras, and engagement, you'll probably spend most of your time on your next project. For most of us, starting a comic involves a lot of scheduling and time management because we're actively working on the comic as it posts (though hopefully with a buffer).