This is shameless self-promotion, but part of a working theory I have that Mastodon users have more to offer Lemmy than your average Reddit user. See my other post about it here: https://lemmy.blahaj.zone/post/2174573
TLDR: Mastodon users are inherently active posters and already understand federation. Also there are MILLIONS of them.
Please consider following if you’d like to get more Lemmy in your Mastodon feed or more Mastodon users in your Lemmy feed!
You can kind of get that on Kbin, though it shunts all the Mastodon stuff over to the “Microblogging” section. Also it’s not exactly bug free right now: a lot of the time you won’t see all of the replies to a post, especially if they’re replies to replies (and so on and so forth).
The idea is there though, and the devs are working on it constantly, so one day it should work correctly.