It was actually the traffic from Google since day 1. Advertising may work for just the first few, but it costs too much to be sustainable. Many podcasts and journalists are startup friendly and would love to recommend you to their following too.
I think conversion is based largely on who it is visiting your site. It's aimed at the nontechnical type of person, so getting traffic from somewhere like HN is going to kill your conversion rate. What are your traffic sources?