Wait for the centralised thing to fuck something up (and they will), then say "hey I've been using this to get away from all the bullshit of [service name]"

In the mean time, post & comment. The more content & discussion there is, the more attractive it will be to others. If you've got a niche hobby you're passionate about, get a community going or try to grow an existing one if it already exists.

I comment way more on Lemmy than I was doing on Reddit towards the end, partly because the people here are generally good to chat to, but also because I want this place to keep being good so I can continue to keep using it.