Don’t target the entire audience you want to reach. By far, it’s easier to start a community with a precise target audience.
If you want to build an online community for teachers, target a very precise group of teachers first. Perhaps Maths teachers in California.
Facebook was supposed to be for Harvard college students. It outgrew that audience and expanded. Outgrowing your audience is the best way to grow.
Many communities launch with too broad a target audience. It’s impossible to build up any pressure. Your audience has less in common. If you fail with one slice of your total audience, you can target another, and another.
Wont this take longer? No. You will build a tighter community at high speed. Facebook certainly aren’t complaining.