It became the only reliable source of information I had. People posted links with a minimal amount of commentary, picking and choosing the best content from other social media networks. They’re not doing it to “build a brand” because that’s not a thing in the Fediverse. It’s too disjointed to be a place to build a newsletter subscription base.
People are more likely to participate if you are. :) But if the space exists, natural discovery can happen too. You do not have to do much more than open the conversation.
I generally agree with activite communities being a self reinforcing feedback loop. That said, one of the challenges federation creates is fragmentation for “the same” community across multiple instances. As a result, each community appears relatively inactive as they’re all vying for engagement with each other.