I've only been on mastodon for a couple of days, but yes it is active, and depending on what you're after it might we worth it. For me, it seems to be a nice mix of keeping up to date with things and people I already know, and at the same time stumble upon (related) things I did not yet knew about.
You will have to put in some effort to follow people/accounts/hashtags, but that's kinda fun. Just try it out!
Main mod here! I won't dispute I have been less active over the last couple of weeks (holidays and other reasons), but I think /c/composting is not doing that bad. It's not high-traffic, but there are posts every few days and I'd say it's not anywhere near dead.
When I created the community, I thought about how much I should post and opted to hold back to see how many other compost enthusiasts would come out of the woodwork. Perhaps I have been a bit too conservative, though?