I think the best way is to pick a niche interest and start there. When I first joined Reddit, it was mostly just LISP programmers and there were no subreddits. They served that niche well, then grew from there. Facebook started out serving Harvard students, then all college students, then everyone.

I started a YouTube channel (https://youtube.com/parttimelarry) last year and was wondering if anyone would find me. I had zero subscribers for my general programming channel, then decided to focus on something I was interested in learning myself. I narrowed my focus to Python for Finance / Automated Trading and documented what I was learning in public, and turns out there are plenty of people looking for information in this niche and they are happy to find a place to discuss it.

Now I am on track to reach 10,000 subscribers this year and have an audience that is very engaged and gives a lot of feedback. So start small, get the first 100 users. Once you have 100 users, you know you can get 1000. And once you get 1000, you feel like you can get 10,000. And then more people start noticing.

I also used to run a message board to discuss the band Tool. It's just one band, but people like discussing their music and lyrics. At the beginning, I needed to be the lead content creator and give people a reason to stick around. They wanted some discussion of lyrics, photos, show reviews, links to news articles etc. Eventually, certain members started posting more than others, like thousands of posts. I made some of them moderators, and they became leaders of the community, and it grew from there. The hardest part is getting the initial 100 or so people who are engaged, are passionate about the topic, and are willing to contribute.