This was driven just by word of mouth and me posting on Twitter. No launch on ProductHunt, Reddit, or the like, given the extremely niche nature of it. No paid traffic either. Just making customers happy with feature improvements and acting on suggestions that we thought were a good addition.
While I don't remember exactly what this felt like, since I'm adding this milestone retroactively in Jan 2020, it was likely a time when we started appreciating how long and slow the SaaS ramp can take. It was discouraging, but as long as we kept things humming along, we assumed word of mouth would continue, and if it didn't, that just meant we weren't doing a good enough job on the product.