1. You should not feel bad about anything. They could fire you at any time (assuming control is split equally among the three of you). You do not owe them anything. As long as you're respectful in telling them you're leaving, there is nothing wrong with quitting.
2. That being said, I also believe that you're a bit misguided about the best direction for a startup at this stage. I disagree that it is smarter to focus on developing an audience instead of building the product. In order to build an audience you at least need an MVP to show to potential customers/users. Your slow audience growth numbers are easily explained by the lack of a minimally fleshed-out product. You are not going to see huge numbers for a landing page. And many products require some sort of education / early adoption before they get mass traction. Speaking for myself, if I see a landing page I'm not going to even bother putting in my email on the invite list. I'd like to see a demo at the very least. The fact that you guys don't have an MVP after two months is a bit of a concern for me. I'm not sure how much responsibility you bear for that. This point is irrelevant as to your decision on whether you should leave, but it's something for you to reflect on if you ever want to start another company (by yourself, with others, etc.).