Back when I was a founder of an early stage startup - who's trying to research and acquire some potential early adopters for our prototype - I tried many channels to get them.
Personal connections, LinkedIn, Reddit, Twitter, Product Hunt, Meetups, conferences, fellowships, incubators, introductions from all our meetings.
That is until I found Slack. Slack was the near-perfect channel for finding early adopters.
It has public groups formed based on interests (e.g., technical writer group, software engineer group, digital nomad group) - this makes targeted attempts quite easy. Furthermore, you can easily DM people based on the context of their comments, the resources they share, sometimes you can even research the person if they put more information about them online. On top of all, most people I have met are very friendly and open to having 1:1 conversations.
Beyond finding early adopters, Slack is also perfect for making friends with strangers with your interests from around the world.