Having tried all three, its a stark difference in how much more social Lemmy is comparatively. Its not even close. Almost all posts I've encountered on lemmy have interaction; whereas, more often than not, posts on the other two platforms have no interaction. Wonder what the driving factor is behind this difference?
mastodon is another "general interest" social media hub along the same vein of reddit or bluesky or .world or .ee, which means that (excluding its founding group) it takes many forms of long term investments to gain sufficient traction enough to establish a core group of active users (assuming that it ever succeeds at doing so at all) and that core group is a small fraction of its user base (presuming that a reddit post i saw years ago showing that a tiny fraction of users on social media are responsible for a vastly disproportionate amount of content on all platforms is true).
lemmy's political origins pre-included the identities and accompanying pre-built core groups that had already start coalescing in other social media platforms like reddit & tiktok. by the time of the reddit blackout protests those groups already had new online safe spaces in various lemmy instances and their ranks swelled at the same time other reddit users started to fill the ranks of other "general interest" instances like .world and later .ee
that link you posted on lemmy user counts reflects the "general interest" instance's difficulties of retaining a core group of active users that disproportionately create the most content. it's around this content is where you will find the interaction/engagement that characterizes lemmy's considerably higher engagement; instead of the news & link sharing lower interaction/engagement that characterizes the "general interest" instances.
right now; the "general interest" instances have a relatively handful of VERY prolific users expending a clearly excessive amount of time and effort at creating a sea of inactive communities & instances in the hopes that it might eventually serve as a basis for a "general interest" core group and i hope that they succeed; i think that the lemmyverse would be better with politically moderate points of view and i'm sure that the "general interest" instances won't lose all of their users to bluesky, threads, nostr, etc. by then.