Built it and they will come.

OK, it was a mixture of things. I told my friends about it and they were sufficiently nerdy to try it out.

I responded to early feedback - specifically about creating a leaderboard. Originally it was all anonymous but people wanted to see how well they'd done.

My wife and I gave an interview to a local BBC radio station which gave it a little bump. Similarly, when it is mentioned on reddit and other sites we pop up and talk about it.

It was also picked up by a couple of academic papers, which gave it a bit of credibility. As did our recent integration with OpenStreetMap.

There's a far amount of schema.org metadata which probably helps with SEO.

But, other than that, who knows? I've had plenty of projects which didn't do as well. Sometimes the Web rewards nice things.