In short: I drove customers poorly and infrequently.

The only marketing I did was through my blog/Twitter. That spawned a fair amount of word of mouth and some other blog posts by people who had used the service. Beyond that I think it was just people talking to eachother because the product really does provide value if you're building a website.

I also have a hunch that the custom thank you videos played a part too but I can't prove it.