It should be self-evident now. It's by far the most active alternative now. It peaked when it began but its now settled into a stable environment. And if it looks like there's a slow overall activity decline (and there is in terms of Lemmy iself due to the slow decline of lemmy.world and the collapse of lemm.ee) - you'll want to also add Piefeds numbers and Mbins numbers into the mix as two alternative software alternatives that speak with Lemmy instances.

Even as Lemmys own development is now slow, they are by far the most developed alternatives with features way beyond any provisional centralised alternative betas that pop up on here. I'd also argue that they are better served for building new communities than Reddit is. Almost every community name on Reddit now is taken, and controlled by others. If you had communities in mind you want to help develop or support, you likely can't. It's all a closed shop. And some of these subreddits are run poorly or flat-out maliciously. Nothing you can do. They have ownership of the name.

The Fediverse doesn't work like this. It's a federated structure. So if a community is poorly run or half-abandoned by the moderators on one instance, it can be ran out of town by simply building it on another instance. This has happened a number of times on-site. I'd also add that the youth of the Fediverse also means there are many more communities up for-grabs by anyone who wants to build them there. There are various support advertisement communities across the Fediverse designed for helping you to promote them. Piefed itself has access to public topics and feeds that allow people to group communities by theme and then get notifications whenever posts are shared to them. Piefed also has post scheduling, flairs, hashtags for promotion purposes. It's just beyond any small alternative that might exist. It is quieter than Reddit, much quieter, but it's by far more active than any other alternative that might pop up on here.

I suppose one caveat here is if you are right-wing, or reactionary and primarily argue politics then the Fediverse is not for you.