You just have to start communities for things you're interested in, and post the kinds of content that you want to see. People from the frontpage will upvote it if it's generally interesting enough (people actually see stuff here), and eventually subscribers will start to submit their own content to sustain the community. Once you get things moving, communities will tend to grow on their own. To be honest, I'm quickly finding out that trying to get people interested in a community from inside of Lemmy leads to quicker growth than trying to get people to switch from an existing platform (like Reddit). I know people are sometimes put off by the idea of deliberate growth, but it's a force of good here on Lemmy. We have to self-organize ourselves before we lose people's interest.

If more people create niche communities and make an attempt to grow them, I think we'll more quickly replace a lot of what we had on Reddit. I mean, what I described above is basically the same growth formula I've used to modest success on Reddit. Heck, even Reddit's own support articles say the same things: create the content you want to see, crosspost said content to other communities, and so on.