A massive group of people don’t suddenly agree to talk to each other. They don’t suddenly adopt the same norms, values and understandings. A massive group of people aren’t going to suddenly want to be in a community.
No, first there are 5 people, then a few more join. Then others follow. Slowly and gradually it builds up into a good-sized community. But it all begins with that tiny group of people first.
If you want to have a large community, you need to have a small community first.
Your first job when trying to develop a community, is to bring those first 5 to 10 people together. You need to reach out to them, explain your vision, earn their trust and then facilitate conversations between them.
That sounds like a lot of effort just to reach 5 to 10 people. But these 5 to 10 people is the foundation of your community, not mass-messages to an uninterested audience.