I'm very surprised it was able to get this much traction despite being launched only 8 years ago, long after the heyday of these sort of sites. How'd you do it?
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.
I'm an OSM mapper & enjoy linking mapped benches to their profiles on OpenBenches. It gets hairy, though, when a single bench features multiple memorial plaques. Wonder if you'd consider revising your data model to permit multiple inscriptions per bench?
I can only hope to be remembered by anyone as fondly as some of these people.