I tried to use the sandbox on the site, but didn't manage to draw anything. I think you should focus on the UI first (make it easier), next promote. If you promote a UI that's hard to use, a lot of people are just going to leave.
Ask a friend/family/anyone to try to use it, and observe them. Don't speak.
We already did the user testing with 3 people. It was painful to watch, sigh. But that was two iterations away and we learned a lot since. Or so we thought.
Were you expecting to actually draw something while being on the site? This seems to be a common gotcha and thing we don't explain well yet.
Maybe we should make it clear that tzigla is not actually an editor. Or maybe we should just plug one in.
Ask a friend/family/anyone to try to use it, and observe them. Don't speak.