While true, I would like to point out who is leaving: The vocal community.

When you see a reddit post and it has 1000 Upvotes and 50 comments, than this means that a couple thousand people saw it, over 1000 votes on it (up and down) and 50 made a comment, and some even commented on a comment. Most people are lurker and are just passive and enjoy the contribution by OP posting it, people curating it by voting for it and giving the topic traction by commenting on it (maybe even provoking another thread of the same topic or adding another thought in another post in the next hours/days or turning it into a meme).

The people, who are leaving - as far I as I see it - are the vocal active people. Not the lurker. So it might not be a mass exodus, but those who are active and vocal about their unhappiness and who are actively searching for alternatives and are now here on Lemmy, are the heart of the buzzing culture of reddit. Those are the ones who bring in new posts, vote actively and comment massively. Not the lurker. So who is left behind on reddit is mostly lurker who are now missing a good part of the active community who commented and voted for them. And I think this is visible on reddit and can accelerate reddits decline.

Its not the mass of the people that is important, but the engaging force that is driving the discourse in a community by being active and vocal.

And I think Lemmy got a good heap of those people.