Just wanted to add my quick 2 cents.
Sébastien and I have been building a productivity app. We were looking for feedback from people that are into productivity and the Getting Things Done method.
So we bought a domain, quickly set up a landing page, posted on our Twitter accounts, and commented through our social networks that we were looking for feedback on the product.
This was an effective way to get about 15-20 people to comment on the product.
In case you're curious, here's the landing page: https://focusd.app
We have also created a free community to discuss and share ideas/workflows with like-minded productivity fans: https://join.slack.com/t/focusdgroup/shared_invite/zt-113gxeetf-kXTPA8Y9NyHVj1Wj3Rdd~A
What also helped to raise initial interest was sharing the journey on our respective newsletters: https://newsletter.dsebastien.net/